Start Firefox in Safe Mode 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

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Restarted Firefox in safe mode, with no add-ons enabled. The issue persists.

You can check the about:sync-log page via the location/address bar to see if there are error logs available. You can temporarily set the services.sync.log.appender.file.logOnSuccess pref to true on the about:config page to also get a log of a successful Sync. Do not forget to reset the services.sync.log.appender.file.logOnSuccess pref to false after you have finished testing Sync. You can upload a recent log to this website if the log is too big to put in a reply.

• http://pastebin.mozilla.org

Try to delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave). If that isn't enough then delete the prefs.js files to reset all prefs and maybe also delete the compatibility.ini file to reinitialize the profile.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox