IE doesn't feature tabbed browsing, which for me is a huge advantage. (I think it does on Mac but I use FF there too).
IE does not display PNG correctly, though they claim it will in ver.7 and you can get some kind of plugin to force it to display them. Seems to me in this day and age the most popular browser should handle PNG and when it does internet graphics will take a big step forward.
My daughter uses IE and I am forever cleaning out the browser highjackers and other malware from her computer. Of course that may have more to do with the browsing habits of teenagers but still, there have been so many security problems with Explorer that I just can't trust it.
As far as speed, maybe they're even but Firefox just seems faster to me at displaying pictures and since I'm a graphics guy this is important to me.
Oh yeah, as far as I know there is nothing like AdBlock for IE and I find it indispensable. In general I think Firefox has better add-ons.
I've heard there are pages that FF can't handle but I haven't seen one yet.