I have not been able to do screen sharing in Firefox or Angelfish (even on desktop screen sharing don’t work in falkon).
Requested screen sharing support in Angelfish at https://invent.kde.org/plasma-mobile/angelfish/-/issues/73
Looks like phosh on Librem 5 screensharing don’t work, though phosh on a laptop worked. Reported it as https://gitlab.gnome.org/World/Phosh/phosh/-/issues/713