There's one old, old, thing IE does that FF doesn't do. It's been bugging me for years, but now I'm gonna say it.
It's the damned, stupid, drag-and-drop behavior Firefox inherited from Netscape.
Here's what I get when I click on a URL in the Firefox and IE location bars, then drag and drop into a blog editor, Microsoft Word, the RTF edit box for in Firefox, anywhere ...
IE uses the page title for the URL text display. Firefox gives me the URL. (I think Safari does the same thing as FF and Camino definitely does the same thing as FF. I don't know about Opera or OmniWeb.)
I'm sure someone prefers the FF behavior. I think the IE behavior is 100 times better. Ok, a thousand times better.
Update 8/1/07: I tried with OS X, turns out there's no drag-and-drop "insert title" behavior with Camino, Safari or (if I remember correctly) - Firefox. So it probably requires some OS cooperation too.