You already know (or at least strongly suspect) what I am going to tell you:

FAKE! MALWARE! RUN AWAY!

It's pathetically easy to create a fake website and while Firefox has a built-in protection against phishing and malware sites, this particular campaign of malware installing sites uses a new address every day, so the bad sites list cannot keep up.

To get new fake update site on the block list: I found a fake Firefox update

To clean a system if you "opened" the "patch": Troubleshoot Firefox issues caused by malware

Also, it appears you run XP. As an extra layer of protection for this older OS that isn't getting regular security updates, consider OpenDNS. A simple settings change adds their blocking feature to your browsing.

https://www.opendns.com/home-internet-security/ (free options)

And finally, since the malware distribution campaign operates through major ad networks, if you don't already use an ad blocking extensions, it might be time to consider one. For example:

https://addons.mozilla.org/firefox/addon/ublock-origin/

Ad blockers can break some site features, so you'll need to develop a habit of keeping an eye on its icon to make an exception in that case.