Yep, the PR [20.09] firefox-wayland: fix screen sharing using pipewire by Stunkymonkey · Pull Request #107699 · NixOS/nixpkgs · GitHub was accidentally merged into nixos-20.09, not release-20.09 or staging-20.09 as would be the correct procedure.
This caused the firefox commit to bypass CI.
It got fixed a few minutes ago, so updating your channel should fix the problem.