Firefox – Using Multiple Instances with Different Profiles

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.

Pass the --no-remote parameter to firefox on startup and it won't re-use the existing process.

This is a known issue with firefox on most unix platforms. What seems to happen is firefox queries the X server, sees that another firefox is running (it doesn't even have to be the same version) and sends it a signal to open a new page, rather than actually starting.

It even happens if the second firefox is started on a second machine over SSH with X forwarding enabled. It's a pet peeve of mine which still hasn't been fixed.

• https://bugzilla.mozilla.org/show_bug.cgi?id=378032
• https://bugzilla.mozilla.org/show_bug.cgi?id=376244
• https://bugzilla.mozilla.org/show_bug.cgi?id=346198
• https://bugzilla.mozilla.org/show_bug.cgi?id=448412