RHEL 10.0 was planned to be released with Firefox and Thunderbird shipping by default as Flatpaks, but there were still some missing pieces wrt. Flatpak related features, Anaconda wasn't able to preinstall RHEL Flatpaks and support for Flatpaks in Satellite was not complete as well. All the major issues are already resolved or will be resolved for 10.2. To switch the default deliverable method we have to update comps with the following example:

      <rhel_only><packagereq type="optional">redhat-flatpak-preinstall-thunderbird</packagereq></rhel_only>
      <centos_only><packagereq type="optional">thunderbird</packagereq></centos_only>

where on RHEL a Flatpak will be used for Thunderbird and an RPM will be used on CentOS Stream as there are no CentOS Stream Flatpaks.