At issue is that this would require compiling against xul, and we
don't actually compile firefox (we ship the official builds
unmodified), so in order to package and provide spice-xpi, we'd have
to build firefox.
I'm inclined to say "no thanks" to that right now :)
I did venture into vinagre's dependencies, and that seems much more
reasonable, and may perhaps also give us RDP and VNC support.
I did, over the weekend, add virt-viewer which is supposed to be able to view spice...