I don’t know when the option was removed since I hadn’t updated Firefox on that machine in some time. Normally it’s in the customize menu at the bottom left. I can confirm that the option still exists in 104.0.2 since I also haven’t updated Firefox on my main machine (a different Ubuntu-based OS). And unless there’s some way in 115.0.1 to remove the title bar then I am extremely reluctant to update. Anyone know how to remove title bars on this update?
If you want to find the bug, you can run a mozregression to find what broke it (using 104 as your last known good release and 115 as your bad release).
You can reuse your profile to test this pretty easily if your issue is more easily reproduced with user data.