Running Multiple Instances of Firefox

With the Sysinternals utility, Firefox on another desktop runs as a different process, just as if you had run it as:

firefox.exe -no-remote

(That usage will also complain if you try to use the same profile twice, even if you're not running Desktops).

It looks like Firefox is using Windows messages to determine whether there's already a Firefox instance running, and doesn't find anything on the same desktop, so starts a new instance.

And this is why you can't use the same profile.

I'm guessing that IE8 works differently (possibly because it uses multiple processes anyway) and uses some communication mechanism that works across multiple desktops.
IE8 probably co-ordinates all access to user (profile) data through a single process; Firefox doesn't, so it will prevent multiple processes using the same profile to avoid the possibility of file corruption.

Alternative

VirtuaWin, available free from Sourceforge, does essentially the same job, and works the way you'd expect with Firefox - multiple Firefox windows across various desktops, all from the same Firefox instance with the same profile.

1. Run firefox -ProfileManager to create a new profile, say test (Note that all Firefox windows must be closed before run this command)

2. Run firefox -P test -no-remote to run another instance of Firefox with the newly created profile

I haven't tried this, but maybe you could could copy and rename your main profile (as many times as you want), then run extra instances of Firefox using these new-but-identical profiles. (Just remember to save any new bookmarks in your original profile or thing could get confusing.)