Firefox is running on the remote, connected to a different X11 session. It is not running on the local. When I use `firefox --new-session --no-remote` on the remote, I see a dialog pop up on my local machine stating Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

Yes, this is because firefox doesn't allow multiple ff instances to use the same profile at the same time. You can ask firefox to create a new profile (`firefox -ProfileManager`) and then use a seperate profile for the remote instance