Are you clearing history when shutting down? That would prevent windows and tabs from last time from being loaded, because that data would be gone. If that is not the case, perhaps your settings are no longer being saved.

• http://kb.mozillazine.org/Preferences_not_saved

When the home page(s) were launched I was able to recover the closed windows by using the history for recently closed windows. Then I would close the instance of Firefox containing the home page. This would make the instance with my tab groups the only one remaining. Then I was shutting that down. Even so, Firefox was launching the home page. I'm a user who likes to keep my histories, but I did clear my cache to see if that would help. It did not.

So, once again I launched Firefox, restored my recently closed windows and closed the home page instance. Then I decided to try restarting with my add-ons disabled. When I clicked on that on the drop-down help menu nothing happened--it didn't restart. However there was a system sound that played. I don't remember which one or what it meant. It was like an asterisk or exclamation.

I shut Firefox down and restarted it manually. Then I restored to the recently closed windows instance and closed the homepage instance. I tried to restart with add-ons disabled and this time it did so. I noticed that I was in the same tab group as when I closed Firefox but couldn't see a way to open the other groups (which I believe is normal behavior).

And so, finally, I shut that down and restarted manually without disabling the add-ons. The same tab group returned rather than the home page and opening the tab groups page showed my normal, desired state. Given that there was a system sound trying to tell me something there might have been something that either was not closing properly or wasn't closing at all. Whatever it was Firefox is now behaving correctly.

I hope this helps you understand what happened. There might be something in the information that was sent that will help, as well. I can't think of anything more but feel free to contact me if you wish.

I have no idea what happened. Sometimes just switching to safe mode and back again seems to fix things. Sounds like one of those quirks that happen occasionally.