IE does it for each different window; Firefox always shares the same instance even for different windows.
As per the title of this entry, you already know that Firefox allows you to do it but the "feature" is simply not available as a simple check box.
1. Start Firefox with the Profile Manager
If you installed Firefox in the default installation location, open a command prompt and run "C:\Program Files\Mozilla Firefox\firefox.exe" -no-remote -ProfileManager2. Create a new profile
You can choose any name but I used "testprofile"3. You are ready now to run a second instance of Firefox with the testprofile
Again, if you installed Firefox in the default installation location, open a prompt and run "C:\Program Files\Mozilla Firefox\firefox.exe" -no-remote -P testprofileIf you need to run the other instance on a regular basis, you will simply create a shortcut with the command line above.
Need to run more than 2 instances at a time? You will need to repeat the process above and create more test profiles.
For more command line arguments, check the Firefox KB.