Is that normal ?
Answers:
Thats because IE uses more resources on your computer than Firefox. If you try to open both browsers and then go to your task list and check your processes, you will see that Firefox uses less memory and resources than IE. IE is just a huge program that does tons of loadup when you have it opened. A lot of people use Firefox and prefer it, but there are others who are just comfortable with IE so they continue to use it.
Isnt the tabbing feature awesome? =)
I.E came up with 7.0 where you can use tabbing too, but the browser itself is SOOOOOOOOOO slow! So its not just you, trust me.
Maybe your Internet explorer went all stupid like it can do if you don't give it updates and virus scans.