I've been going from IE to Firefox and back again as I get annoyed by one of them.
I have favoured FF of late because it was faster and opened a group more easily.
But FF bugs me as it doesn't open PDF's easily, and it usually won't play web videos.
IE does both, and I've now upgraded to IE9, which looks really good, so i'll stay on that and see how it goes.
FF also had lots of bugs and faulty versions; as well you needed quite a few add-ons, which can cause problems.
IE seems to be better quality deep down.