Does the same thing happen when you run Firefox in Safe Mode? An add-on might be causing it, such as Classic Theme Restorer (not sure, just a guess). If that works, run the regular Firefox mode again and disable add-ons one by one to find the culprit.

Also make sure you don’t have a userChrome.css file in your profile, or that it has been set up properly - see this question for more info.

Thanks. I've had Classic Theme Restorer ever since they made weird decisions about where browser buttons go, show/don't show, etc., a long, long time ago. Always saw the change in tabs when new notices came in.

It's weird that online searches only show people asking to turn this feature off. I find it invaluable. I couldn't reverse engineer a solution from reading how to turn it off. This happened recently, that it stopped working, maybe 1-2 updates ago.