wrapFirefox: support GDK_BACKEND=wayland

The firefox wrapper now supports setting the GDK_BACKEND to wayland
which is useful in cases where firefox would be started from within an
X-Application inside of wayland. GTK/GDK would otherwise default to the
X11 backend in those situations.

The intention is that people that are using wayland primarily pull in
the new `firefox-wayland` top-level attribute into their environments
instead of just `firefox`. Firefox will then always be started with the
correct rendering backend.