How do I launch a remote firefox window via SSH?

besides firefox -no-remote another parameter is firefox -no-xshm which reveals the technique used to make it work.

X11 shared memory is an interprocess communication technique which can be used by all applications connected to a given x server session. It can be used to perform drag & drop, and other kind of desktop interaction.

It can be (and is) used also to implement "open once" applications, in order to reduce the footprint (or the number of windows).

Since the X11 protocol is network transparent the "shared memory" is extended also to remote X11 clients.

Try firefox -no-remote

Note, I did dome digging as this was bugging me and you can also just add:

MOZ_NO_REMOTE=1
export MOZ_NO_REMOTE

to your profile.