Mozilla Firefox issues

"Firefox is already running"

Symptoms of this issue are the lack of a browser window and display of the message "Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window.".

If the folder is empty and Firefox displays an error on startup - it is sufficient to delete the file once.

Simultaneously launching two Mozilla Firefox processes on two different computers using the same account is not possible.

Check if a firefox.exe process is already running and in that case end it. Make sure that you're not logged in on another computer simultaneously. Also check if you have enough free space on the disk J: - in case of exceeded quota Firefox also won't launch.

If you still cannot launch a browser, the solution is deletion of the parent.lock file from current browser profile located at J:\.AppData\Mozilla\Firefox\Profiles\[your Mozilla Firefox profile name]\. If you have no idea what is the name of your browser's profile and you absolutely don't care about it, you may delete the whole J:\.AppData\Mozilla\Firefox\ directory, but you'll lose all personal settings and plugins installed.

"Changes to Firefox profiles"

Symptoms of this issue are launching a Firefox window with a clean profile (without add-ons, bookmarks) and a message about changes to profiles in Firefox. This is a direct consequence of the official change to profile management made by Mozilla with Firefox 67.

The fastest way of reverting to the old profile is:

• closing all Firefox windows
• right clicking on the Start Menu -> Run
• firefox -p
• selecting the old profile, most likely named default, as the default profile