Tabbed browsing. nuff said.

I use IE but have used firefox and IE has a much better tabbed browsing setup to be honest.

If Opera was a woman, I would romance her and make love to her. The mouse gestures are the most brilliant feature to me; being able to sit back and open and close tabs and move all over the place by wiggling my mouse is great. I ditched Firefox for it because I found Firefox took ages to start. Opera is very zippy from my experience.

Firefox had it first and, thanks to ultimate reason #2: plug-ins galore, you can customize it however you like and much better than IE.

I also notice that Firefox loads my webpages faster.