When replying to the forum post, only the single post to which you are replying is displayed on the screen (below the editor). But this often doesn't provide enough context to write the reply. You need to see some of the other posts too.

Showing the whole thread would slow down the page unnecessarily, so that's not the right solution. What I find myself wanting to do is pop up the thread in another window so that I can browse or copy from it.

The problem is that there isn't a link to the thread anywhere on the reply screen. I have to open up a new browser window/tab, find the forum, then find the thread again. There absolutely needs to be a link to the thread on the reply screen.