1. (Uppercase) Mozilla Firefox, the browser of choice among furfags. Firefag has many fanboys, much like the Playstation 3, which is another over-hyped piece of shit only furfags and Canadians would like.
2. (Lowercase) Someone who uses Firefag, insisting that "it's better than IE because it has tabbed browsing" even though IE has had tabs since the introduction of IE 7, but firefags will insist "oh, they stole that from Firefox," even though Firefag stole it from Opera, which likely got the idea from AOL.
Firefags are typically smug, elitest, anime-fan type basementdwellers, just like most Mac users, and like to try to force other people use Firefag, much like how Macfags like to force people to use Macs (Professors at art college, for example). Sometimes, they will even go so far as to make their websites only work when viewed on Firefag, but that begs the question: If Firefox is so great, people would choose to use it of their own volition, so why do you have to force others to use it?
3. (Lowercase) A retard.
Any annoying, condescending prick who goes out of his way to make it known that he uses Firefox for all of his web browsing needs.
Fark poster #1: OMG, that link had a very annoying pop-up ad.
Fark poster #2: LOL, n00b. I haven't seen a pop-up ad in over two years, thanks to Firefox.
Fark poster #1: STFU, Firefag.