If you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should NEVER respond to such an alert to avoid getting infected with malware.

• only update Firefox via "Help -> About" or by downloading and installing Firefox from the Mozilla and never via a pop-up or link on a web page.

See also:

• https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update
• https://support.mozilla.org/en-US/kb/avoid-and-report-mozilla-tech-support-scams

Thanks Cor-el. Some additional digging suggests that the issue is associated with an ad on the page. (I have ad-blocker, so I don't see any ads, but somehow, this fake update request got through.) I also have Norton security active, and that and/or Firefox itself occasionally blocks potentially harmful pages, and that wasn't protection wasn't triggered. So, do you think the webpage itself (which relates to parks in Cozumel) is safe, and it's just this pop-up that's sketchy? (I'm planning on making a reservation online at one of the parks.)