Finally got screensharing working in Firefox on Guix + Wayland thanks to @dthompson (and @daviwil and @abcdw !) Here's what I had to do:

- Use this wrapper https://git.dthompson.us/guix-config/tree/dthompson/home/pipewire.scm#n61
- Wrap firefox https://git.dthompson.us/guix-config/tree/dthompson/machines/ikaruga.scm#n158

Thanks y'all!