How can I launch two Firefox windows under the same profile?
Solution 1:
This is because Firefox uses some sort of locking mechanism so that only one Firefox process can access each profile, and I think that's to avoid corruption in the sqlite databases (places, bookmarks, history, etc).
If you start Firefox with -no-remote, it claims exclusive access to the profile ("default" in your case) and no other Firefox process can use it, even if the second instance is invoked without the -no-remote.
You can of course use Ctrl+N or File > New Window to create a new window from within Firefox.
The proper way around this is to run Firefox 4 with -no-remote, and invoke your Fx3.6 instances without that flag.