This epic will track the work that is required to rebase Firefox ESR packages in RHEL from version 128 to 140. For the common information (i.e. schedule) about Firefox ESR and Thunderbird ESR rebases see the parent feature - RHELBU-2928. Process related tasks are tracked in DESKTOP-1485.

Summarized changelog - for more information please look at https://en.wikipedia.org/wiki/Firefox_version_history#Firefox_128_through_139 and changelog for 140 release itself:

• Firefox 129 was released on August 6, 2024. Major changes included the addition of an enhanced Text and Layout menu in Reader View with new options for character spacing, word spacing, and text alignment; a Theme menu with additional Contrast and Gray options, with the ability for the user to select custom colors for text, background, and links from the Custom tab; the display of a Tab preview when hovering the mouse over background tabs, making it easier to locate the desired tab without needing to switch tabs; the replacement of HTTP with HTTPS as the default protocol in the address bar on non-local sites; Address Autofill for users in France and Germany; added support for more inactive CSS warnings, including cases where the resize and float properties used incorrectly, box-sizing is used on elements that ignore width/height, and table-related CSS properties are not on table-related elements; the ability for the Network Blocking feature in the Network panel to block HTTP requests in addition to blocking responses; the Rules side panel in the Inspector panel displaying @starting-style rules, as well as a tooltip for the var() function, indicating the @starting-style CSS custom properties value; the Rules side panel showing the impact of invalid at computed-value time custom property declarations in the computed panel; added support for querying the encryption key system config in mediaCapabilities.decodingInfo() API; added support for Float16Array typed arrays along with new DataView methods for reading and setting Float16 values, and a Math.f16round() static method that can be used to round numbers to 16 bits; added support for @starting-style, which allows to define styles that are applied to an element when it is first rendered, enabling transitions on elements that are added to the DOM or that have their display type changed from none to another value; added support for the transition-behavior CSS property; added support for the textInput event; added support for DNS lookup of HTTPS Resource Records (RR) with the native DNS resolver, increasing HTTPS coverage and facilitating the use of Encrypted Client Hello (ECH) if present in HTTPS RR; the resolution of HTTPS DNS records with the operating system's DNS resolver on specific platforms.

• Firefox 130 was released on September 3, 2024. Major changes included translating selected text portions to different languages after a full-page translation; an easy way to try experimental features with a new Firefox Labs page in Settings, such as the AI Chatbot feature letting users add the chatbot of their choice to the sidebar for quick access as they browse, and the Picture-in-Picture auto-open experiment enabling PiP on active videos when switching tabs; the enabling of overscroll animations as the default behavior for scrollable areas on Linux; a fix for an issue where Copy and Paste context menu items intermittently were not enabled when expected; WebCryptoAPI now supporting Curve25519 primitives (Ed25519 signatures and X25519 key generation); the Web Codecs API being enabled on desktop platforms, allowing low-level access to audio and video encoders and decoders; added support for the Catalan, Croatian, Czech, Danish, Indonesian, Latvian, Lithuanian, Romanian, Serbian, Slovak, and Vietnamese languages.

• Firefox 131 was released on October 1, 2024. Major changes included an offer to temporarily remember when users grant permissions to sites (e.g. geolocation), to be removed either after one hour or when the tab is closed; the display of a tab preview when hovering the mouse over background tabs, making it easier to locate the desired tab without needing to switch tabs; the ability to take into consideration languages previously used for translations when suggesting a default translation language; the re-introduction of the ability to navigate to the search engine home page when the search bar is empty by using shift-enter/shift-click; the Tab overview (List all tabs) menu's new, refreshed icon; the removal of SVGGraphicsElement.nearestViewportElement and SVGGraphicsElement.farthestViewportElement as part of the ongoing updates to SVG 2; added support for text fragments, which allows users to link directly to a specific portion of text in a web document via a special URL fragment; the rejection of SameSite=None cookies when there is no Secure attribute included; added support for Cookies Having Independent Partitioned State (CHIPS), allowing developers to opt a cookie into storage partitioned per top-level site; and support for the Swedish language by translation.

• Firefox 132 was released on October 29, 2024. Major changes included WebRender hardware accelerated rendering enabled for most SVG filter primitives, improving performance for certain graphics-heavy content; the ability to block third-party cookie access when Enhanced Tracking Protection's Strict mode is enabled; the blocking of HTTP-favicons if they can not be received over HTTPS instead; the Copy Without Site Tracking grayed out when no known tracking parameters are found within the link; the addition of more tracking parameter support for websites such as LinkedIn and Shopee; the removal of support for HTTP/2 Push due to compatibility issues with various sites; console logging in service workers being functional again; restored support for debugging remote devices via USB; added support for a post-quantum key exchange mechanism for TLS 1.3 (mlkem768x25519) which secures communications against advanced / long-term threats; added support for Certificate Compression which reduces the size and increases the speed of a TLS handshake; text directionality computation updated to follow the latest model defined in the HTML specification, improving interoperability with other web browsers; the availability of the requestVideoFrameCallback() method on the HTMLVideoElement interface, which enables developers to perform efficient operations on each video frame; the getCapabilities method allowing applications to gather the media capabilities supported for the live MediaStreamTrack; the fetchpriority attribute enabling web developers to optimize resource loading by specifying the relative priority of resources to be fetched by the browser; the disabling of the heuristic granting storage access for some window openings

• Firefox 133 was released on November 26, 2024. Major changes included a new anti-tracking feature, Bounce Tracking Protection, which is now available in Enhanced Tracking Protection's "Strict" mode; the sidebar to view tabs from other devices that can now open via the Tab overview menu; the "Picture-in-Picture: auto-open on tab switch" feature from Firefox Labs behaving more reliably across a wider range of sites, automatically opening relevant videos while ignoring others; the adjustment of the "expire" attribute value by adding the difference between the server and local times when server time is available; added support for the keepalive option in the Fetch API, which allows developers to make HTTP requests that can continue to run even after the page is unloaded, such as during page navigation or closing; added support for the Permissions API in Worker Context; the dispatch of beforetoggle events just before a dialog opens and toggle events after the dialog closes, matching the behavior of popovers; the availability of methods on UInt8Array to convert to and from Base64 and hexadecimal encodings, which is an implementation of a Stage 3 TC39 proposal; added support for image decoding as part of the WebCodecs API

• Firefox 134 was released on January 7, 2025. Major changes included added support for touchpad hold gestures on Linux; the expansion of Ecosia's availability to all languages in the German region along with Austria, Belgium, Italy, Netherlands, Spain, Sweden and Switzerland; the ability to follow the model HTML specification for transient user activation more closely, which makes popup blocking less strict in cases where previous versions were overly aggressive, reducing erroneous blocking prompts; a refreshed New Tab layout being rolled out to users in the US and Canada, featuring a repositioned logo and weather widget to prioritize Web Search, Shortcuts, and Recommended Stories at the top; improved support for debugging web extensions, such as automatically reloading the web extension's source code in the Debugger when the extension is reloaded; Debugger log-point values automatically converted into profiler markers, making it easy to add information to the marker timeline directly from the Debugger; the Network panel displaying information about Early Hints, including a dedicated indicator for the 103 HTTP status code in the user interface.

• Firefox 135 was released on February 4, 2025. Major changes included added support for the Simplified Chinese, Japanese, Korean, and Russian languages in Firefox Translations; the credit card autofill feature gradually rolled out to all users globally; AI Chatbot access gradually rolled out to all users; the enforcement of certificate transparency, requiring web servers to provide sufficient proof that their certificates were publicly disclosed before they will be trusted (only affecting servers using certificates issued by a certificate authority in Mozilla's Root CA Program); the gradual rollout of the CRLite certificate revocation checking mechanism, substantially improving the performance of these checks; the inclusion of safeguards to prevent sites from abusing the history API by generating excessive history entries, which can make navigating with the back and forward buttons difficult by cluttering the history; the option to close only the current tab if the Quit keyboard shortcut is used while multiple tabs are open in the window for Linux and macOS users; improvements to the Translations feature which will reduce the likelihood that models will invent new, made-up words under some circumstances; the availability of the refreshed New Tab layout in all countries where Stories are available; the removal of the "Do Not Track" checkbox from Preferences; the rename of the "Copy Without Site Tracking" menu item to "Copy Clean Link" to help clarify expectations around what the feature does; the provison of Linux binaries in XZ format, replacing the previous BZ2 format, offering faster unpacking and smaller file sizes; the display of a warning when content-visibility is used on elements where size containment does not apply; the introduction of a new console command $$$ that allows searching the page, including within shadow roots; the enhancements to WebExtension debugging, where workers are now available in the Console panel's context selector and breakpoints function correctly in content scripts; added support for a post-quantum key exchange mechanism (mlkem768x25519) for HTTP/3; the attribute values which indicate the coordinates of PointerEvent that may now be fractional values rather than only integers, allowing web apps to handle the events with higher-precision coordinates when the target element is transitioned by CSS and/or the viewport is zoomed; the change in behavior of mouseenter, mouseleave, pointerenter and pointerleave events for improved spec compliance when the last mouseover or pointerover event target is removed; added support for the WebAuthn getClientCapabilities() method.

• Firefox 136 was released on March 4, 2025. Major changes included the option for users to enable the updated Firefox sidebar in Settings > General > Browser Layout to quickly access multiple tools in one click, without leaving their main view; the new vertical tabs layout; the Clear browsing data and cookies dialog that allows clearing saved form info separately from browsing history; Smartblock Embeds that allows users to selectively unblock certain social media embeds that are blocked in ETP Strict and Private Browsing modes; the upgrade of page loads to HTTPS by default and the fallback gracefully to HTTP if the secure connection fails; the enabling of hardware video decoding for AMD GPUs on Linux; the Weather forecast on the New Tab page expanding to additional regions, including Mexico, Brazil, Argentina, and Chile, as part of an ongoing regional rollout; address autofill enabled for users in the United Kingdom; the preference of the PNG format when copying images out of the browser, allowing the preservation of transparency; the move of the Save to Pocket action from a button to the context menu along with other actions for New Tab stories, such as Bookmark; the Developer Tools debugger editor using Codemirror 6, which improves performance; added support for the Intl.DurationFormat object, which enables language-sensitive duration formatting; added support for the CSS :open pseudo-class for styling elements that can be toggled "open" to display more content; added support for the :has-slotted pseudo-class, allowing authors to style the contents of a <slot> element when it is not empty or not using the default value; added support for the CookieStore API, an asynchronous cookie API for scripts running in HTML documents and service workers; added support for ARIA elements reflection; the browser sending a referrer from meta refreshes and Refresh headers; added support for sending and receiving the AV1 video codec over WebRTC; added support for sending multiple simultaneous versions of the same source over WebRTC, commonly called simulcast, with the H264 video codec; the value plaintext-only that can now be specified for the contenteditable attribute, making the raw text of an element editable but without supporting rich text formatting.

• Firefox 137 was released on April 1, 2025. Major changes included the rollout of tab groups; the Firefox Address Bar Refresh 2025 with a unified search button, search term persistence, secondary action buttons, contextual search mode, contextual search engine options, and intuitive search keywords; the identification of all links in PDFs and their conversion into hyperlinks; support for HEVC playback on Linux; the ability for the user to sign PDFs without leaving the browser; the use of the address bar as a calculator; the display of the fonts metadata in the Inspector Fonts panel; the ability of the Network panel to override network request responses with local files; added support for the SVG 2 path API; added support for the hyphenate-limit-chars property, which enables authors to have greater control over the use of automatic hyphenation (desktop).

• Firefox 138 was released on April 29, 2025. Major changes included the introduction of profile management; the ability for U.S. users to trigger weather suggestions in the address bar; the availability of Tab Groups, a new way for users to organize and take control of their tabs; the ability for users to copy links from background tabs using the tabstrip context menu on MacOS and Linux; improved address and credit card autofill experience to better handle forms that update dynamically as users input information; the rededication of previous color settings to Contrast Control settings; added support for the import map integrity field, allowing users to ensure the integrity of dynamically or statically imported modules; implementation of support for Error.isError, enabling brand checks to determine whether an object is an instance of Error; added support for the error.captureStackTrace extension to improve compatibility with other browsers; a new column added to the Network panel to display the full path of the request URL; the application of a uniform user agent (UA) style to <h1> elements, regardless of whether they are used inside <article>, <aside>, <nav>, or <section>; added support for Import Attributes, allowing developers to specify additional metadata when importing modules; the ability for applications to set their preferred degradation method to be used when the configured frame rate and resolution cannot be maintained; enabled cache directive for the Clear-Site-Data header, allowing websites to clear the network cache, which gives websites more control over stored data, such as clearing the cache during logout to help mitigate privacy risks

• Firefox 139 was released on May 27, 2025. Major changes included the availability of Full-Page Translations within the extension pages that start with the moz-extension:// URL scheme; the availability of the New Tab custom wallpaper (and colors) option; the availability of Link Previews as an experimental feature which can be enabled via Firefox Labs in the settings; the PNG images keeping their transparency when pasted into the browser; the improvement of the upload performance of HTTP/3 particularly on resumed connections (QUIC 0-RTT) and high-bandwidth and high-delay connections; the shutdown of the Review Checker feature on June 10, 2025; the preservation of the filter setting in the Network panel across DevTools Toolbox sessions; the Debugger's directory root now scoped to the specific domain where it was set, which aligns with typical usage and avoids applying it across unrelated domains; the refined appearance of the paused line in the Debugger for better visibility, especially in high contrast mode; the Temporal proposal, a better version of Date, enabled by default; support for timer throttling for Workers; closed <details> elements that are now searchable and can be automatically expanded if found via find-in-page; window.getSelection().toString() now correctly returning the text serialization when text is selected in a text control, improving cross-browser interoperability on some sites; added support for the WebAuthn largeBlob extension; added support for requestClose() to HTMLDialogElement; the built-in editor for contenteditable and designMode now handling collapsible white-space(s) before block boundaries and white-space sequences between visible content more consistently with Chrome

For even more summarized changelog between 128 and 140 take a look at https://www.mozilla.org/en-US/firefox/140.0esr/releasenotes/ (not available yet).