I can't access the profile manager in firefox 4 the same way in 3.6

Have you tried using "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -P ?

I had the same problem in linux and after trying out new profile manager app (https://developer.mozilla.org/en/Profile_Manager) i tried the switch -no-remote and got it working ;)

for example: firefox -p invented -no-remote

PS: profile invented does not exist :P