How to use Firefox 12 and above on Desktops?

With versions of Firefox up to and including 11, I was able (using 2 different Firefox profiles) to open two instances, once on Desktop 1 and one on Desktop 2. With Firefox 12 and above, this no longer works. That is, Firefox opens its second profile window just fine – BUT ALWAYS ON DESKTOP 1. I therefore cannot use it as I always have, on several screens at once (which gives me a hugely increased desktop area).

Does anyone know how to fix this, please? I imagine there’s something in Firefox’s Config file that can be changed but I’ve failed to find it. Thanks!

(To clarify, this is sysinternals desktops.exe. I am on Windows XP, SP3.)