That would be a Scam. Close the tab and move on.

hello, mozilla will never send out such kind of popups - this is a scam trying to trick you into downloading more mailcious stuff. in case you're getting such kind of pop-ups regularily though, this by itself might be a symptom that there is already adware/malware present on your system.
in this case please try the following:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to firefox > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

Whenever you get a message / popup that software / files need to be updated,

DO NOT USE ANY OF THE PROVIDED LINKS.

While this may be a legitimate message, it could also be Malware or a Virus. Any time you want or need to check for upgrades, go to the web site of the True Owner of the program in question. For example, to check out Firefox, go to Mozilla.org.