At yesterday's WebSG meetup, someone asked me how you can have 2 different user sessions in Firefox, and I was unable to answer as I never had the need for 2 different user sessions (though I imagine it'd be nice for testing certain types of interactive web applications).
Anyway, I found the answer today after some Googling - Lifehacker's "Manage multiple Firefox profiles" article.
Basically, once you have started Firefox in a profile that you want, you can start another profile by passing the '-no-remote' switch together with the name of the profile you want to start with the '-P' switch:
/path/to/firefox -P profile_name -no-remote
It does mean you need 2 different profiles though, for the 2 different user sessions. Oh well.