Extendability, security, and stability. Ok, it crashes sometimes, but almost all the time, restores quickly and easily. Ad-block is a huuuge plus. Customizability. Works on all OSes. First to do tabbed-browsing, which I can't live without (exaggeration). Fast. Undo close tab! Personas.

I haven't used all other browsers, to be honest, but I like my firefox.