[gerwim]

If you create a new profile for the different user, you can start a second session:

firefox -new-instance -P "Another Profile"

[JonathanBeuys]

I don't have (nor want) multiple users.

I just want to open a new window for my one user.

[TheCapeGreek]

So what's wrong with Super+N? I'm struggling to see the need for a completely new browser instance when you only want another window.