I have a Linux machine running all the time and sometimes want to login to that machine via "ssh -X ..." from another Linux machine and start firefox. Due to the fact that there is already running a firefox on that machine, I always get the the error that there is already an instance of firefox running. What I want is that just a second window of the already running firefox is started (like pressing Ctrl+n) and displayed on my remote machine. I have already tried -new-window and -no-remote, but this did not help.