Hmm, first of all, not being an XP user, IE 7 will not run on my system for sure ...
But yes, though firefox is a better browser overall, both have their share of +ve & -ve points. This in short is my view -
Mozilla Firefox
+ Open source and cross platform. Its biggest advantage. It is available for Linux(X-Windows), Windows and many other platforms. Coz of its being open source, you have the moox build and other mods that enhance it.
+ More features, extensions, good for the frequent surfer who needs a better organised and convenient way to browse.
- Slow, bloated, rather too heavy for what it does.
IE
+ Light to load, good browser if you don't surf too many pages at once
+ Compatible with most sites (this is the fault of site owners, who "optimise" their site completely for IE)
- Insecure
- Low on features (the only reason its so light)
- Heck, its only present on few operating systems. I would rather not browse internet in windows, thank you.
But heck, either way I'll choose Opera