This is a scam tactic that is trying to trick you into installing malware, so don't download or execute this kind of stuff just because of a popup or redirect on a website! Firefox Updates are handled automatically by Firefox so you don't have to download anything for that and you can always initiate a manual check for updates in the Firefox menu ≡ > Help ? > About Firefox.

That is coming form "malvertising" (=ads placed on legitimate websites that redirect you to this fraudulent message), so using an Adblocking addon like uBlock Origin would help with blocking that garbage.

I found a fake Firefox update

caz1111 said

Hi, It appears to be malware. if so, perhaps it is prudent to publish a notice? I just wanted to let you folks know to protect others using your browser.

Unfortunately this is not a case of a few sites staying up for a long time (till they are taken down) in trying to trick less experienced Windows and or Firefox users in downloading and running this fake firefox-patch.js file before scanning it. The scammers behind this use disposable sites so they are only used for a day or so to host this fake urgent Firefox update page (also a similar fake one for Chrome) and then use another site the next day.

The scammers made the fake urgent Firefox update page look somewhat legit to distract people from the fact that they are not on a official *.mozilla,org page.