Then go to Add/Remove Programs, scroll down to "Mozilla Firefox" and remove it, choosing to keep your bookmarks, customizations etc., (don't checkmark the box).
Then reboot and delete the folder called "Mozilla Firefox" at this location: C:\Program Files\Mozilla Firefox
Finally run the installation file you downloaded to the desktop earlier.
Your bookmarks, customizations etc., are maintained in a different location and will become available to you again once you complete the installation.
As a precaution, make a backup of your Bookmarks as follows:
Press CTRL+SHIFT+B to open the Bookmarks Manager.
Click the link to Import and Backup and then click Export HTML and save the file somewhere.
You can use this file at any time to restore your bookmarks should that prove to be necessary. Simply reverse the procedure and choose "Import HTML instead.
N.B. If you use a download accelerator, disable it before you attempt to download Firefox. These utilities are a frequent cause of extraction failures.
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