Using another computer, you can download a "full" installer for Firefox 39 from the following page -- scroll down to your preferred language: https://www.mozilla.org/firefox/all/ Or there is a way using components of Windows that I have seen in other threads if the "plumbing" of IE still works. I can't recall the details, but someone could search them up if you do not have access to any other systems. Also, you might try the Windows System File Checker as long as you're making repairs: https://support.microsoft.com/en-us/kb/929833