Allegedly, on or about 19 October 2016, Rick Stevens sent:
I've done this before. Essentially:
1. Ensure "X11Forwarding yes" is set in the REMOTE system's /etc/ssh/sshd_config file and make sure sshd has been restarted to read it. 2. Run "xhost +" as root on the LOCAL system to permit everyone to make connections to the LOCAL system's X server. 2. "su - unprivileged-user-name" on the LOCAL machine. 3. "ssh -X user@REMOTEHOST" to log into the REMOTE machine. 4. Verify you have a DISPLAY variable set by doing "echo $DISPLAY" on the REMOTE machine. You should get a value like "localhost:10.0", indicating you have X forwarded. 5. Run "firefox" on the REMOTE machine. The display should pop up on your local machine.This has worked for me for a long time.
It's an issue with Firefox, chiefly. It tries to be clever, and use an already (locally) running Firefox instance if there is one.
Sure, if there isn't any Firefox running at the time, then issuing the command will run Firefox on the remote computer displaying on the local machine. But, if if Firefox was already running locally, then any attempt to run Firefox, will (usually) re-use the local one.