Don't make any changes on the Safe mode start window. 

"Clear the Cache": Edit > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now" "Remove the Cookies" from sites that cause problems: Edit > Preferences > Privacy > Cookies: "Show Cookies" Start Firefox in Safe Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). https://support.mozilla.com/en-US/kb/Safe%20Mode Don't make any changes on the Safe mode start window. See: Troubleshooting extensions and themes https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes See also: Cannot log in to websites https://support.mozilla.com/en-US/kb/Cannot%20log%20in%20to%20websites Check and tell if its working.