Windows' System Restore usually doesn't break Firefox, but it usually doesn't fix it either. I recommend backing up your Firefox settings in case something goes wrong during the next few steps. See [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information]. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.) Next, try reinstalling Firefox 5 over the existing installation. http://www.mozilla.com/firefox Does it work?