Seems to be possible to fix that with the configuration option --enable-default-toolkit=cairo-gtk3-wayland like it is done on the newer official builds. I don't know how this changes the build requirements.
Don't know if you want to do such a change in stable. Maybe you just want to add a check for GDK_BACKEND. Then you can add --no-remote in /usr/bin/firefox That changes behaviour. But only in the case when it would crash. So I think no one would complain about that. Maybe besides displaying a proper warning.
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