See: *http://kb.mozillazine.org/Profile_in_use *https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding It is possible that your firewall or other security software blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program. Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full, unrestricted, access to install for Firefox and the plugin-container process and the updater process. See: *https://support.mozilla.org/kb/Server+not+found *https://support.mozilla.org/kb/Firewalls *https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating Try to start Firefox in <u>[[Safe Mode|Safe Mode]]</u> by holding down the Shift (Mac: Options) key to see if that helps.