Full screen mode doesn't work (black screen)
When I trying to enter the full screen mode (pressing f11) I only seeing black screen.
Alla svar (2)
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.