I've been using Photoshop since 2007, and i was thinking that fireworks have something special than Photoshop, but once PS CS5 is released, i changed my mind !
Without getting into all the the technical (vector, raster) stuff, think of it this way, Firefox is designed more towards web only graphics plus it has better compression, where PS is designed to be used for all types of media graphics. When it comes to IM, I would think that it would not make much of a difference which one you were to use. I choose PS only because I have been using it for years.