I may be wrong, but I believe there would be conflicts between Firefox and Iceweasel, due to Iceweasel still using the Firefox desktop entries and user settings folders.
Also, why do you need two broswers, and, more importantly, why would you need two versions of essentially the exact same browser (aside from the name and artwork, exactly the same)? Why wouldn't you just open more than one window of any one browser? (You do know they can still do that, don't you?)
Having two different browsers (ie. Firefox and Chrome) is all well and good, one browser may do things slightly different/better than the other, but having two of *the same browser* (aside from naming and artwork, ie. Firefox and Iceweasel) seems kinda pointless.
Then again, if you need to be logged into the same site on two different accounts, I could see an argument for two different browsers, however, Iceweasel and Firefox wouldn't work for the reason I stated above (using the same user settings folder). You would need to create a separate profile for each version, in which case you could still get away with only one version installed anyway and just launch two different sessions of it using two different profiles... (Someone else would have to go into detail about how to do that, I don't offhand know since I'm only one user on one computer and only need one profile... Some information here
, here (though it's written for Windows)
, and here
might be useful...)