I am on elementaryOS Hera and have an interesting phenomenon where some blackish bar occludes part of Firefox's viewport:
Also you can see that the viewport is cropped somehow (see the beige help box to the right). It is not part of the webpage, and opening Developer Tools leads to another black bar at the bottom:
The only things I did recently where enabling webrender in
about:config (reverted, no change) and running
sudo apt install mesa-* (since reverted, no change).
Firefox Version: 73.0.
What I've tried
Starting FF in safe mode, without plugins, removes the problem. However, disabling manually all my addons does not help, so it must be something else about safe mode that does it.
prefs.js preferences also does not solve the issue.
What could be causing this? FWIW, I have an Nvidia RTX 2070 Super.