Be careful when using this fix as deleting any other files in your profile can corrupt the User Profile.
If you receive the error “Firefox is already running, but is not responding.” and have followed this post, it’s possible that your Firefox profile is still “locked”.
To remove the locks:
- Ensure Firefox is not running (not even in the task manager) before you do this or you could potentially corrupt your profile.
- Open your Firefox profile folder by Pressing Windows key and R, then copy-paste the following line into the box and press Enter.
%appdata%\Mozilla\Firefox\Profiles
3. Double-click the profile folder ending in .default. The profile has a random prefix name on each computer, but it always ends in .default.
4. Locate the parent.lock file and delete it.
Firefox should now launch properly.