I use Firefox because of the security and plugin features. I love tabbed browsing - don't know how I lived without it for so long using IE. If a site is IE centric then I have a plugin that lets me open that site with the Microsoft browser whenever I type it into Firefox. The popup plugins for Firefox work like a charm too. I haven't so much as seen a popup since I started using them....I hate popups as much as anyone....it really is a life saver.
I do wish there was a way to save my browsing when I close Firefox, like Maciamo mentioned about Opera....that would be icing on the cake.