hi CeliaA, unfortunately 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! 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).

we suspect this is coming form "malvertising" (=ads placed on legitimate websites that redirect you to this fraudulent message), so using an adblocking addon like https://addons.mozilla.org/firefox/addon/ublock-origin/ would also help with that.

I found a fake Firefox update

Thanks so much -- I just downloaded the addon. I probably should have mentioned that these started after I had to migrate to a new pc last week. I think I either had the addon you linked to or another on the old system. Anyway, thanks again for the really quick response - I really appreciate it!