I tried posting this reply as a Quick Reply, but it didn't take. I don't know why. Unfortunately, I wrote my message directly into the Quick Reply box, and now it's gone.
So I'll try again with the regular reply feature and hope for better luck.
I have Firefox, Opera, and IE installed on my Windows XP Pro computer, and as others have already said, there's no problem, and you can use all of them, even simultaneously. Indeed, Firefox has an add-on available called IE View
that enables you to see the same page in IE. If I come upon a page in Firefox that I want to see in IE, I simply right-click and select View this page in IE. IE opens to the same page. Firefox also has an add-on called IE Tab
that will open a page using the IE rendering engine in a separate tab. I might add that Opera has a "view in IE" feature built into it. So yes, you can certainly use IE and other browsers, even at the same time.