No. Not legitimate.

On a Windows PC you can check if your Firefox is up to date by selecting the "Help" menu (ALT-H) and choosing the "About firefox" option. While displaying various version information Firefox will check if it is up to date and provide a method to get an update if necessary.

Mozilla doesn't issue "paches" for Firefox, that's always done by releasing a new version that some call a "minor" version release and are known as "chemspill" releases. And example is the Firefox 46.0.1 release; the 0.0.# (second period & number in the version number string) indicates that it was a "chemspill" release.

Also, you didn't mention what file type was 'offered', but Mozilla uses .mar files for updates - not .exe files which only work on Windows operating systems. So theta is a tip-off that it ain't genuine. The .mar file type is unique to Mozilla and it works for Windows, Mac, and Linux operations systems.