community/firefox: Add screen sharing with Wayland

Enable pipewire WebRTC supported using a patch from Firefox' flatpak version [1]
which allows screen sharing with Wayland.

Note: This requires the packages `pipewire`, `xdg-desktop-portal`, and some
`xdg-desktop-portal-<BACKEND>` to be both installed and running to work.

[1]: https://github.com/xhorak/firefox-devedition-flatpak/tree/master/org.mozilla.FirefoxNightly
7 jobs for !8438 with firefox-pipewire in 3 minutes and 54 seconds
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