If the problem persists, you can reset Firefox and start afresh.
The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.
To Reset Firefox do the following:
Go to Firefox > Help > Troubleshooting Information.
Click the "Reset Firefox" button.
Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
Firefox will open with all factory defaults applied.
Did this fix your problems? Please report back to us!
If you would like to take a listing of the add-ons, changed preferences/settings etc. to quickly replicate the previous profile, please Copy all to clipboard (on the left of the reset button), and paste it into MS Office Excel, or OpenOffice/LibreOffice Calc, or notepad. However, it would be a good idea to check for the existence of the issue immediately after the reset.
You can also check for extension that may be hiding content if you have any.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.