You can try to disable hardware acceleration in Firefox.

• Settings -> General -> Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to make changes effective.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Thanks for the suggestions.

There is no available update for the graphics display driver.

I also tried disabling hardware acceleration and the recommended performance settings. Initially, this worsened things. After restarting the system and upon opening the browser it lagged a lot until it froze. Restarted again and same behavior. After the third restart though, and after enabling hardware acceleration and the recommended performance settings back again, the menu and tab bar unexpected behavior are gone.

I must say this "going randomly back to normal" had already happened before today. Back at work the described bugs started, three restarts + disabling the extensions solved it for the rest of the session. Then, at home I turned the computer on and the problems were back. Then I wrote this post here because I can't figure out what is causing them for the life of me.