If you want two instances of Firefox running, say one for your normal browsing and the other to host “web applications” such as Hoot Suite, or your blog open all the time and pinned to a desktop/space, this will work.
First: commands for opening up another running instance of FireFox from the command line (The example here is given for Windows, but applies if adjusted accordingly for Mac OS X.) : http://kb.mozillazine.org/Opening_a_new_instance_of_your_Mozilla_application_with_another_profile
Another more complicated method here: http://mabouali.wordpress.com/2011/07/18/opening-multiple-instances-of-safari-or-firefox-on-mac-os-x/
Second, in Mac OS X, using the Automator: http://stackoverflow.com/questions/281372/executing-shell-scripts-from-the-os-x-dock
Or Apple Script (actually uses FF as an example): http://superuser.com/questions/16750/how-can-i-run-an-application-with-command-line-arguments-in-mac-os
Son of a gun… seems like the “assign to” feature would still apply to both instances/profiles.