I saw this discussion in the mozillaZine Firefox Builds forum today in reference to Bug 836758 - Convert Panorama into an add-on and remove it from Firefox. It took me a minute to recall what the heck was Panorama/Tab Groups. Personas came to mind but then that was themes, not tabs. Then it hit me, it was this promising feature for power users (or people who like to have a lot tabs open) introduced in Firefox 4, but then it became disabled by default in future releases.
There is still a small user base for this feature who have been using this since it was introduced. The purposed add-on would be installed (or the user prompted) if tab groups are detected once this feature is removed from the core Firefox build. So in theory, current tab group users wouldn’t lose their settings.
Not sure what to think of this. On one-hand since the user base is so small may be it is better to be add-on, but if the developers hadn’t hidden this feature right after releasing it two-years ago may be it would have a larger user base. The latter which may have given Mozilla justification to develop and enhance this feature.
Reviewing the bug there is currently no Target Milestone (version) specified. We are still a couple weeks away from the next merge which means this could still end up in the current nightly for Firefox 21 (due May 14, 2013) but likely won’t see this until Firefox 22 (due June 25, 2013).