PipeWire should be enabled bu default - but it's enabled in Wayland sessions only: https://searchfox.org/mozilla-central/source/third_party/libwebrtc/webrtc/modules/desktop_capture/desktop_capturer.cc#80
Bug 1648029 Comment 5 Edit History
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PipeWire should be built by default - but it's enabled in Wayland sessions only: https://searchfox.org/mozilla-central/source/third_party/libwebrtc/webrtc/modules/desktop_capture/desktop_capturer.cc#80