Beware, Firefox doesn't always do what it says on the tin. There are some web sites you will not be able to view with it. You'll find that anyone answering a question relating to browsers on here are nearly all Firefox fanatics.
Well i've tried all the popular ones and Firefox is no better for the average Joe than any other and worse in some cases as it doesn't allow viewing of all web sites. Avant, though based on IE is just as good, just as customizable and just as safe with Vista. Make your own mind up alinic and don't let these guys brainwash you. Unless of course you're a web developer or the like in which case I don't suppose you'd be asking this question.
Occasionally I find a site that doesn't work well with Firefox, but believe it or not, since getting IE7, I've found more sites that have problems with IE7 than with Firefox.
As Ethel says, there are add-ons that allow you to switch to IE if you hit a problem with a particular page (I use IE view lite), but generally speaking, if a site's worth using it's almost certain to work in Firefox. I wouldn't swap for the world!
If further testimony is required all I can say is Firefox is my preferred browser , and I have tried many over the years. IE is not as user-friendly or as fast as Firefox when using FasterFox 2.0.0. addon.