Sorry, this error occurs when the updater didn't completely replace old files in the program folder. Firefox can't run properly with a mix of versions. To solve this problem, you can download and run the "full" installer. It should detect your existing installation and redo the update. You can get that download here: https://www.mozilla.org/en-US/firefox/all/#product-desktop-release Ref. https://support.mozilla.org/en-US/kb/firefox-wont-start-find-solutions#w_xulrunner-error