Hi Jason, one of the things often included with Sync are add-ons. So with that in mind: Could you test in Firefox's Safe Mode? In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem. ''If Firefox is not running:'' Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.) ''If Firefox is running:'' You can restart Firefox in Safe Mode using either: * "3-bar" menu button > "?" Help button > Restart with Add-ons Disabled * (menu bar) Help menu > Restart with Add-ons Disabled and OK the restart. ''Both scenarios:'' A small dialog should appear. Click "Start in Safe Mode" (''not'' Refresh). Any improvement? ---- That error message also can arise when there is a problem with a local configuration file, although I am not aware that Sync could copy it over. To investigate: Check for alien script files in your Firefox program folder as follows. Those are nearly always in one of these locations (varies for 32-bit / 64-bit): * C:\Program Files\Mozilla Firefox\defaults\pref * C:\Program Files (x86)\Mozilla Firefox\defaults\pref Caution: ''Do not double-click script files!'' The default action for a script file it to execute as a Windows system script. (Typically this would just not work, but why risk it.) Make sure Windows is showing hidden files: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files It's also a good idea to turn on display of all file extensions: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/ A file named '''channel-prefs.js''' is normal. '''Any other file''' in this folder is suspicious and could be used to modify Firefox behavior. Remove any such files to a neutral location for further analysis at your leisure. You can rename the file with a .txt extension and examine it for the name of another file that contains the locking code.