In my opinion as mainly a Firefox user that switched from IE, Firefox is my favorite because it feels faster and more reliable than IE and knowing that my browser has the ability to be openly compatible an expanded on with addons.
It has more useful features that were present before MS started copying similar features in IE7.
I feel like I have more control over the options in the FF browser and can configure it in more ways if I wanted. That feeling of freedom is what made me switch to Firefox.

I think the feeling of freedom is what people have when they use Firefox.
In my opinion, I think this a big part of what made it really popular.

Nowadays, I only use IE during web development to just make sure my javascript is compatible in IE.