firefox: screensharing broken

Nixpkgs version

• Unstable (25.11)

Describe the bug

When trying to share my screen or select windows I can only select "entire screen", which in turn is just a black screen.

dbus-monitor only shows firefox trying to inhibit org.freedesktop.ScreenSaver, but nothing related to ScreenCast (very little lines in general)

google-chrome on the other hand properly starts everything, and its calls for stuff like org.freedesktop.impl.portal.desktop.cosmic to get path=/org/freedesktop/portal/desktop; interface=org.freedesktop.impl.portal.ScreenCast; member=SelectSources; everything works properly

Steps to reproduce

1. start firefox
2. go to https://mozilla.github.io/webrtc-landing/gum_test.html
3. observe

Expected behaviour

same as in chrome; screen gets shared

Screenshots

Firefox:

Chrome:

Relevant log output

Additional context

I am using the COSMIC DE, I have tested screensharing in google-chrome and zoom-us , both working as expected.

this bug also happens in firefox-esr and firefox-beta

the only other machine i tested with is running arch linux + kde, there screensharing works

System metadata

• system: "x86_64-linux"
• host os: Linux 6.12.55, NixOS, 25.11 (Xantusia), 25.11.20251028.08dacfc
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.31.2
• nixpkgs: /nix/store/p6gvrb36n5dh0cdg6d77rkpapy5kc0gq-source

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Reproduce the failure in Firefox using the linked WebRTC getUserMedia test, then observe D-Bus traffic with dbus-monitor while comparing Firefox with google-chrome on COSMIC. The issue is done when Firefox can select a window or screen and share it successfully, including in firefox-esr and firefox-beta where applicable.