Starting Firefox with a specified profile from the dock (Mac OS X)
It's a bit more work to do this on mac but this is what you want:
Managing Multiple Firefox Profiles in OS X (via Internet Archive WayBackMachine)
The custom Firefox icons are still available from Gnome-Look.org
Script Editor based solution was quite slow for me, so I looked for an alternative.
I ended up creating an application bundle with a bash script inside - see details at Using Firefox profiles on Mac OS X.