It’s easy in Chrome, just open a new “incognito” window and you’re set. Firefox however has to switch the whole application into Private Browsing mode. And since it is designed to allow only one copy of the application to access a profile at the same time it is not a trivial matter to just open another window.
So here’s how you do it:
- First create a new profile using something like the following:
firefox -no-remote -ProfileManager
- Then use that profile to start a Private session next to the regular one:
firefox -no-remote -P newProfileName -private
You could check the firefox command line reference for more information.