Apple actually does not use real FirePro cards in their MacPro. They're using normal consumer cards labled as "FirePro" to get some more $$$ out of them. E.g. their D700 card is just a rebadget, downclocked HD 7970.
To answer your question: Some FirePro cards are supported OOB, others need some more tweaking. To get an idea about the support state, you'll have to find a similar consumer-level AMD card and look up the support state:
http://www.tonymacx86.com/graphics/171291-radeon-compatibility-guide-ati-amd-graphics-cards.html
Finally, you'll have to find out the device ID of the FirePro card and verify if the Apple drivers include it. If it's present, the drivers will pick the card up automatically, otherwise you'll need to spoof the device ID.
If you have a specific FirePro card in mind, I can assist you, but be aware that you can't be 100% certain about compatibility until you find someone who is sucessfully running the card in a Mac.
Additionally, please be aware that there's not a single advantage of getting a FirePro card in OS X, so unless you need it in a dual boot Windows setup, there's no reason to get one. The OS X performance is inferior to consumer cards due to decreased clock rate and the price is usually way higher.
