hello, this is a scam tactic that is trying to trick you into installing malware, so don't download or execute this kind of stuff! only download firefox from its official location at mozilla.org - in addition updates are handled automatically by firefox so you don't have to download anything (you can initiate a check for updates in the firefox menu ≡ > help ? > about firefox).

in case this is caused by rogue advertisements on legitimate websites, you could install an adblocking extension in firefox: https://addons.mozilla.org/firefox/addon/ublock-origin/

you might also want to run a full scan of your system with the security software already in place and different tools like the free version of malwarebytes, adwcleaner & kaspersky security scan in order to make sure that there isn't already some sort of malware active on your system that triggers these false alerts.

Troubleshoot Firefox issues caused by malware

swampwander said

My browser opened with 'emergency update' which my antivirus immediately flagged. Is the update fake or real?

Even when a Firefox update is done on short notice for say security reasons, Mozilla still calls it a Firefox update. No mention of words like urgent or emergency is used.

No it is not legit. The fake updates exe can install things like trojans, viruses or unwanted software based on past reports.

The desktop Firefox is not just for Windows as it is for Mac OSX and Linux also so .exe would not be an effective way to send out Firefox updates. The updates are done internally in Firefox (with a .mar type of file) or by download from mozilla.org like say www.mozilla.org/firefox/all/

Even if Mozilla were to use .exe for Firefox updates on Windows, they would be serving them from a *.mozilla.org url and not from random websites with weird names.

A screen shot example of a fake Firefox update/patch page is at https://support.mozilla.org/en-US/questions/1129282