TBB --allow-remote mixes up with plain Firefox

I am trying to use TBB with --allow-remote (see #15185 (closed)) and Firefox at the same time, but both get mixed up.

Steps to reproduce:

both regular

1. /path/to/tor-browser/Browser/start-tor-browser
2. /usr/bin/firefox
3. firefox torproject.org -> opens in firefox
4. start-tor-browser torproject.org -> "Tor Browser is already running, but is not responding."

allow-remote, tbb first

1. start-tor-browser --allow-remote
2. firefox -> opens new TBB window
3. start-tor-browser --allow-remote torproject.org -> opens tab in TBB
4. firefox torproject.org -> opens tab in TBB

allow-remote, ff first

1. firefox
2. start-tor-browser --allow-remote -> opens new FF window
3. start-tor-browser --allow-remote torproject.org -> opens tab in FF
4. firefox torproject.org -> opens tab in FF

It seems both somehow call just plain "firefox" and take the first one running on the system, without checking and distinguishing instances.

start-tor-browser from line 341 calls ./firefox and sais this:

# --class and --name parameters are used to make sure WM_CLASS is set
# up correctly, to identify itself from plain Firefox windows (and
# prevent from mixing up with them).

Looks to me like WM_CLASS doesn't work as intended?