Edit: I stand corrected by the post immediately following this one.

Sorry, on Mac's you can only have one version of Firefox installed at a time. I guess that is a limitation of Mac OSX because multiple versions are possible on Linux and Windows.

There is a Mac user's group that makes a Portable version of Firefox, but it doesn't look like they have a 4.0 version ready yet.
http://www.freesmug.org/portableapps/firefox/

Sorry, but this isn't true. You don't need to use the Portable version to run multiple versions of Firefox on Mac OSX. You can find instructions at this link. It works the same way running multiple versions works on Windows.

http://www.businessinsider.com/how-to-run-firefox-4-and-3-simultaneously-in-mac-os-x-with-multiple-firefox-profiles-2011-3

For web development testing I have multiple versions of Firefox running on my Mac right now.

Can't you add -P "profile" after the firefox-bin command line to start Firefox with a specific profile after you have created the new profile?

• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox
• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile