Probably not legit.

There is some malware going around that seems to promoted in website ads. The browser will navigate forward to a tab with a Firefox Logo, possibly on an orange background, saying there is an urgent update or patch, and pushing a download.

Don't take that bait.

Check out the address bar, it often is some gobbledygook that was just registered last night and will be abandoned tomorrow as a ploy to avoid the built-in security measures that block untrustworthy sites.

Firefox 48 is out now, but you'll get the usual subtle kind of message about that, not displayed in a web page. If in doubt, cancel all update messages and use the About Firefox dialog, as described in this article: Update Firefox to the latest release.

So you are getting a random name website page claiming to have a so called urgent update for Firefox and a prompt to download a fake firefox-patch.js file. The random name of the website alone should raise a flag that it was not legit.

This is not from Mozilla or the Firefox web browser. The fake firefox-patch.exe and firefox-patch.js files can install things like trojans, viruses, unwanted software or to download additional stuff onto Windows based on past reports if the user runs them.

The Firefox updates are done internally in Firefox (with a .mar type of file) whether on Windows, Mac OSX or Linux or by download from mozilla.org like say www.mozilla.org/firefox/all/

You could try using a adblocker extension like uBlock Origin to block theses fake ads if you keep getting them. https://addons.mozilla.org/firefox/addon/ublock-origin/

Unfortunately this has gone on for several weeks now with one or two new sites reported almost everyday. https://support.mozilla.org/en-US/forums/contributors/712056/ and https://support.mozilla.org/en-US/forums/contributors/712075