Firefox Update Malware
This notice to download an update to Firefox IS NOT a legitimate update file. It is a Firefox Update Malware designed to trick you in to installing a virus on your computer.
Under no circumstances should you allow your computer to open this file. When presented with the option, you should choose “Cancel” and then close out the browser window.
If the problem persists, you should schedule a consult and have your computer looked at to verify that you do not have any malware present.
NOTE: All updates are handled automatically by Firefox so you don’t have to download anything. You can initiate a manual check for updates in the Firefox menu ≡ > Help ? > About Firefox