On Ubuntu 20.04, I don't know if the problem started when updating Firefox v81 or Nvidea graphic card driver v450.
Issue: The Firefox show transparent window and pages random flickering. The desktop and other apps aren't affected. In Preferences tab, I'm using "Recommended Performance Settings" on, but nothing change if set it off.
One tip: I opened the black screen of VLC player behind the Firefox window to achieve see something on transparent screen.
Some idea to fix it?