So far, there is no immediate support for that. Either find out how to pass the profile of your choice via the command line, and launch Firefox through a custom command line, or find out what Javascript code is needed to switch a profile and tell me...

Update: firefox profile command line tells me that https://developer.mozilla.org/en/Command_Line_Options has the information:

-P "My Profile"
[download]

or

-profile "/path/to/profile"
[download]