FWIW, I just did a clean install of Plucky using the daily ISO from here which was surprisingly uneventful 
After installing libcamera-tools, qcam worked out of the box:
$ sudo apt install libcamera-tools
$ qcam
What seems to be missing to get Firefox to play with the camera is the libcamera plugin for pipewire. Installed that plus helvum for debugging:
$ sudo apt install pipewire-libcamera helvum
You need to reboot (or maybe logout/login is enough) or restart pipewire/wireplumber for this to take effect:
$ systemctl --user stop wireplumber
$ systemctl --user stop pipewire
$ systemctl --user start wireplumber
Now run helvum and check that the camera is visible (Built-in Front Camera):
I haven’t tried the camera with the Firefox snap but I doubt that’s working. So I removed the snap:
$ sudo snap remove firefox
$ sudo apt purge firefox
$ echo '
Package: firefox
Pin: release o=Ubuntu
Pin-Priority: -1
' | sudo tee /etc/apt/preferences.d/ubuntu-firefox
The last command above makes sure it doesn’t come back…
Then installed Firefox from the Mozilla repo by following this guide. As mentioned previously, it’s required to browse to about:config and flip media.webrtc.camera.allow-pipewire to true.
With all that, the camera is (somewhat) working when browsing to gUM Test Page.