I suppose it's a matter of opinion and I don't want to get into a flame war with anyone here.
I switched from Internet Explorer to Firefox because it was plain boring and lacked features, I then switched to Opera because it's more lightweight (Firefox is a 4.7MB download while Opera is a 3.6MB download) and has more features. Whilst I can see how extensions can be a bonus, Opera has so many features I don't need any, they can also slow Firefox down and potentially pose and additional security risk (extensions aren't 100% safe, some can even break firefox).
I've tried IE7 and it's alright I suppose, I like the user interface and (unlike Firefox) it has proper page zooming but it lacks support for some web standards (mainly SVG (Scalable Vector Graphics) (like WMF but better)) which I feel should be commonplace nowadays (and would be if it wasn't for IE).