Profile support for ffox on the mac is a bit opaque, but I use it to keep a completely separate environment for e.g. online banking.

From the shell ".../Firefox.app/Contents/MacOS/firefox-bin -ProfileManager" lets you create new profiles, and then ".../Firefox.app/Contents/MacOS/firefox-bin -ProfileManager -P ProfileName" will launch firefox with a particular profile.

When you launch ffox this way, you can have multiple copies of it running at the same time. I don't really expect choosy to know what to do here automatically (prompt maybe?).

However, when you quit any ffox instance Choosy loses track of the other running ffox processes and no longer shows a check next to it in the menu. This I consider a bug... I have to exit and relaunch either choosy or ffox for it to start sending urls to ffox again.