FoxyProxy adds command-line arguments to Firefox so that you may use different settings than when you last exited firefox.
The argument is -foxyproxy-mode, and has this signature:
-foxyproxy-mode [patterns|disabled|<proxy-id>1|random2|roundrobin2]
When NOT specified, FoxyProxy resumes from the last mode in which it was used. When specified, FoxyProxy starts in the specified mode.