You can try "Clear the startup cache..." via the "Help -> More Troubleshooting Information" (about:support) page. *https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox You can create a new profile as a quick test to see if your current profile is causing the problem. See "Creating a profile": *https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles If the new profile works, you can transfer files from a previously used profile to the new profile, but be careful not to copy corrupted files to avoid carrying over problems. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

Try signing out of your sync account. Are there any errors in the Web Console (Ctrl+Shift+K) or Browser Console (Ctrl+Shift+J)?