Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0 In your More System Details it appears that you are using a Ubuntu supplied package version of the old Firefox 50.0.x Make sure the old Firefox 50.0 is not running when you try to start Firefox 66.0.2 Also make sure you are starting the firefox script in the Firefox folder for Fx 66.0.2 and not using a launcher or shortcut that is pointing to Fx 50.0 https://support.mozilla.org/en-US/kb/install-firefox-linux If you are the only user then putting Firefox folder in ~/Home will make it easy for read/write permissions for Firefox updates.