Where did you see this? A new update was released a few days ago.. If you have auto-update on, Firefox will automatically update itself. If not, please do update to the latest version by going to 'About Firefox'.

I was reading a page on http://www.warhistoryonline.com/ when this warning suddenly appeared, and an attempt was made to download and install a new "Firefox security update". I have automatic update turned on, and the whole thing looked rather fishy to me. It also didn't seem to come from Mozilla, but from site called "Firefox update". I exited out of the site and began checking Mozilla for information. Since I see nothing on the Mozilla site, I have to assume this is a phishing hack, trying to load some type of malware to my computer.

That does sound weird. Please type in "about:support" without quotes in the address bar and tell me what 'Version' number you are seeing. I do see that you are running 43.0.1. The latest version is 43.0.2

If possible, please go to 'About Firefox' by clicking the menu button, then the question mark button which is also the help menu. There Firefox should automatically download and once it's done, will ask you to restart Firefox to apply the update.

For a couple months now some sites have had a popup for a fake Firefox security scan to then try and convince inexperienced Windows and or Firefox users into downloading a Fake firefoxpatch.exe

This .exe will then install CryptoLocker, a ransomware trojan if you run it on Windows.

If you downloaded this file do not run it and instead make sure it is deleted.

Since the desktop Firefox is also for Mac OSX and Linux and not just for Windows, the Firefox updates are done internally in Firefox (with a .mar file) or by download from www.mozilla.org or www.mozilla.org/firefox/all