Good point.
I use Firefox for several different reasons.
FF is highly customizable, with multitudes of extensions that add all kinds of tweaks and functionality... plus they're free.
Mozilla also has a host of downloadable themes for FF... which are also free.
Since FF is open-source, many people have developed extensions and themes outside of the official Mozilla lists, and have them hosted all over the web... and they, too, are free.
FF is open-source (as I said), which means if someone finds a security hole or some dipstick designs a virus to attack FF, there are several thousand people (if not a higher multiple) who immediately jump into action to work out a fix to protect what they love and deem as their own. These people do it for the sake of keeping FF safe, and they do it for free.
FF itself is free. Big plus to me. And it was free the whole time it has been around.
FF is not put out by Microsoft. It is not integrated with my OS. It was not shoved down my throat whether I wanted it or not. I chose to download FF of my own free will.
And those are some of the reasons why I use Firefox.