Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":
- Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
"Remove Cookies" from sites causing problems:
- Tools > Options > Privacy > Cookies: "Show Cookies"
Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).
- Don't make any changes on the Safe mode start window.