F11 will allow you to enter full-screen and pressing F11 again, will allow you to exit it. While in full-screen, you will not see the tab bar. Spoiler There were some add-ons, which were not popular, which also removed the tab bar, but they were not maintained and discontinued (last updated 2015).
No it's not that. It was something in the about:config page and it removed it. It was like everything got shunted up slightly and the web bar was right at the top where the tab bar is now
I'd love to know what it was. I cannot find anything concerning hiding the tab bar even going back as far as Firefox 40, which was released August 11, 2015