Firefox is a web browser. It’s not all that rare these days to have a perfectly competent user of the Internet ask "What is a web browser?". This boggled my mind for a long time, since they use one every day. But it occured to me that if Ford were the ONLY maker of cars, no-one would call them cars, they’d call them Fords.
So, when you want to look at a web page, you use a web browser. Most of the world uses Microsoft Internet Explorer, primarily because they don’t know that there are better offerings. Many people will remember Netscape, which was the leader for a long time, and is now gone.
Firefox is what has come out of the history of Netscape. It’s free, it’s powerful, it’s extensible, it’s fast, and it’s secure. Internet Explorer is none of those things, really. Some will argue that it is, but it’s not. 🙂