Try reading [[Websites look wrong or appear differently than they should]]

Clear the cache and the cookies from sites that cause problems. "Clear the Cache": *Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now" "Remove Cookies" from sites causing problems: *Firefox > Preferences > Privacy > Cookies: "Show Cookies" Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> 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. *https://support.mozilla.org/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes