the problem is that currently firefox refuses to let you run multiple copies on one display. it detects that there is already a copy of firefox on that display and instead of starting a new copy, it sends a message via X for the existing copy to open a new window to the URL instead.
I have no problem with firefox defaulting to checking to see if there's already a copy running as this user and having that copy open a window, but what it does today is that if you have a copy running on a different system, as a different user (on a different network that you may not be able to talk to except the X display), trying to start a new copy of firefox will instead instruct the copy on the remote machine to open a new window instead.
this also means that you can't have two VMs each open a browser, unless you don't use X for your display and instead use somethign even less efficient like VNC