Hello ,

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING:Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Please report back to say if this helped you!

Thank you.

Did you check for issues with the parent.lock file?

See also:

• http://kb.mozillazine.org/Profile_in_use
• https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

Are you using "Clear history when Firefox closes" to clear personal data like cookies?

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"

You can check for issues caused by plugins and set all plugins to "Ask to Activate" on the "Firefox/Tools > Add-ons > Plugins" page (plugins are not affected by Firefox Safe Mode)

• https://support.mozilla.org/kb/Troubleshooting+plugins

Thanks for the suggestion. I did exactly as you suggested, went into the C:/ drive and deleted Mozilla then reinstalled from my Download directory. Same problem, same error comes up. Is there a more advanced solution?

An update : This problem arrived when I started up this week after my holiday. Having looked at the system again, I have rolled back to before a large batch of Windows Updates that uploaded when I turned on the first day and discovered that had been some kind of problem with my User directory. I have now managed to delete Mozilla in that version and reinstall it and everything is now working. I will now do the updates again manually and check what is happening. Thank you to both Brandon Vita and Cor-el for coming back with suggestions so quickly.