Needed packages are up, you can help with testing by installing with:
sudo pacman -U http://kaosx.tk/repo/build/pipewire-0.2.4-1-x86_64.pkg.tar.xz http://kaosx.tk/repo/build/xdg-desktop-portal-1.0.3-1-x86_64.pkg.tar.xz http://kaosx.tk/repo/build/xdg-desktop-portal-kde-5.14.4-1-x86_64.pkg.tar.xz
Next edit .desktop:
kate /usr/share/applications/firefox.desktop
and make the exec line read like this:
Exec=GTK_USE_PORTAL=1 firefox %u