Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).
If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:
Click the menu button
, click Help
and select Restart with Add-ons Disabled.
If Firefox is not running, you can start Firefox in Safe Mode as follows:
On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
On Mac: Hold the option key while starting Firefox.
On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
When the Firefox Safe Mode window appears, select "Start in Safe Mode".