A noticeable difference between Firefox and Internet Explorer 7 is the find on page feature. Firefox offers a very nice ‘status bar’ type interface that is unobtrusive as you search for a specific word or phrase on the current page. IE7’s is a bit more antiquated as it is a cumbersome dialog box which can interfere with you being able to see the full page. A simple solution to this is the free extension, Find As You Type for IE.
This extension replaces the dialog box (Ctrl + F) with a tool bar interface and offers similar functionality to Firefox’s. I use the search functionality quite a bit and this add-in makes the two browsers consistent with their behavior.
I wouldn’t say this is a must-have add-on, but if you use both Firefox and IE7, I would definitely recommend it.