Does that happen in Firefox safe mode? If not, it could be an extension causing the problem. * http://kb.mozillazine.org/Websites_look_wrong

Many site issues can be caused by corrupt cookies or cache. * Clear the Cache '''[https://support.mozilla.org/en-US/kb/how-clear-firefox-cache?redirectlocale=en-US&redirectslug=clear-cache-delete-temporary-files-fix-issues How to clear the Firefox cache]''' {web link} and * Remove Cookies ''This will log you out of all sites you're logged in to. '' Press the '''<Alt>''' or '''<F10>''' key to bring up the tool bar. Followed by; Windows; '''Tools > Options''' Linux; '''Edit > Preferences''' Mac; ''application name'' '''> Preferences''' Then '''Privacy.''' Under '''History''', select Firefox will '''Use Custom Settings'''. There is a button on the right side called '''Show Cookies'''. If there is still a problem, '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''' {web link} While you are in safe mode; Press the '''<Alt>''' or '''<F10>''' key to bring up the tool bar. Followed by; Windows; '''Tools > Options''' Linux; '''Edit > Preferences''' Mac; ''application name'' '''> Preferences''' Then '''Advanced > General.''' Look for and turn off '''Use Hardware Acceleration'''. Poke around safe web sites and see if there is still a problem. Then restart.

This sounds like a problem with hardware acceleration in Firefox. You can try to disable hardware acceleration in Firefox. *Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available" You need close and restart Firefox after toggling this setting. *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes