Over the last few years Mozilla has continued disable the chrome css code I use to put the Tab bar just below the Address Bar & Bookmarks Bar.
They have done it again with Update 89. I disabled the proton view from the beginning [ at the intro screen ] and I only have a single tab [current one] that shows at the bottom no matter how many are open.
It gets blocked by the Task Bar at times and also covers other Firefox info that I have displaying. It also moves across the page from left to right as I have more tabs open.
I see a number of people on the tubes who have tried to fix this one with mixed results, but I'm a little short on time to try each one to see if it will work on any of my computers.
It would be good if all the open tabs showed as they used to with a little border and an indicator for the current tab.
Maybe its what you have done before, so possibly only a few customizations are needed to correct your current settings.