Flashing visible/invisible windows, mostly at start up.
Sometimes when starting Firefox ("developer edition", running on Ubuntu 20.04, Gnome shell, X11 windowing system) it appears 80% full screen -ish, but actually the window is full screen size - you just can't see it. It flashes really fast, and doesn't repaint correctly. It's very hard to get rid of it as you're never sure what you're clicking on.
Restarting it a few times and I get lucky and it works. I think this problem has gotten worse over the last few months.
I believe it might be something to do with the anti fingerprinting, but I'm not sure. It's making it unusable for several friends, but I'm a bit more stubborn so I'm looking for a fix.
I used to run into this error on an older (normal beta) version (but on the wayland compositor). You may have luck reinstalling Firefox. To do this, you will need to remove it, then reinstall it.
I also would suggest not using developer edition because of it's nonexistent package manager integration, as the beta is on the same version and can be updated with the rest of your system. If you do this, your current profile is probably named dev-edition-default.
You can create a new profile as a quick test to see if your current profile is causing the problem.
See "Creating a profile":
If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.