I'm running a Firefox shortcut with the flags
-P default -no-remote
to launch Firefox 3.6 under the "default" profile. I do this because I'm running Firefox 4 under a "beta" profile, and with different profiles, I can run both Firefox 3.6 and 4 simultaneously. If I don't have different profiles, opening one Firefox opens another instance of the current running Firefox (if I have 4 open, and I try to launch 3.6, it will open another window for 4).
Before I added the profile flag, I could open a million Firefox 3.6 windows under the same profile. Now I can only open one window, and subsequent windows say "Firefox is already running, but not responding."