You can open Firefox on Mac OS X with a certain profile with a command line like:

open -n /Applications/Firefox.app --args -P some_profile -no-remote http://localhost:9011/test_user

The URL is on my dev server, it creates a Facebook test user and then redirects to it. First you get an access token from graph.facebook.com/oauth/access_token, then you use https://graph.facebook.com/APP_ID/accounts/test-users to create an account using the token you just got.