What OS? Try downloading a separate copy of Firefox and run it from the folder(do not sign into you Firefox account) to see what happens.

https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Linux, Manjaro distro with KDE on it. I tried to do what you suggested and nothing changed. But, I tried to make my taskbar always visible (it had auto-hide property) and now full screen mode is working. I had been trying other browsers (with auto-hide property turned on) and all was working.