How do I fix a corrupted firefox.exe file?

A power failure can corrupt program files, so I’d try reinstalling Firefox first. You can download the latest installer from Mozilla and install it over the existing copy, which normally keeps your profile and bookmarks.

If it still won’t start, I’d uninstall Firefox and reinstall it, but don’t delete your Firefox profile unless you know you have a backup. That’s where your bookmarks, history, and settings live.;