Manually delete the Firefox folder in Program Files. Restart your computer then download the Firefox installer from here - http://www.mozilla.com/en-US/firefox/all.html - using a different browser.

Then install Firefox. Your Firefox personal data should survive the deletion and re-installation, as that data isn't stored in the Program Files folder.

A locked padlock overlay on icons on the desktop or in Windows Explorer means you're sharing that file with other users.

1. Use Internet Explorer to download the Firefox installer from
   • https://www.mozilla.org/firefox/all/
2. Delete the Firefox installation folder, which by default is located at
   • C:\Program Files (x86)\Mozilla Firefox
3. Run the installer you downloaded earlier. When you next start Firefox, it should work normally, complete with your personal data like bookmarks.

If the problem persists, see if the following article helps:

• Firefox won't start - find solutions