Firefox. If you are in dought about changing or not try this might help you...In general, multimedia/it people uses Firefox...probably none uses IE. So, there must be many reasons for that.I build websites and I can tell you that IE is a pain...If I build a website for firefox it will look the same in any browser (apart from IE). If I build it for IE it might look different in every other. So, IE is the one that does not follow the rules.
Do you use tabs on IE? Do you know that tabs already existed on firefox for a couple of years before IE? So, what you are finding useful now you could have been enjoying for years. That is just one of the features. With firefox "there is an app for almost everything" (like the iphone) :)
Hope this help.
You can have both on your computer...I do, but IE is just for testing sites. Don't give a miss to Google Chrome either...try it out...it is fast and simple...I love it.