Some notes from out-of-band discussion with Tobias:

There do not appear to be AppArmor DENIED messages in the journalctl
logs, so the issue may not be with the snapd "camera" interface missing
permissions for libcamera.

The evidence is that the firefox deb works works with the webcam and the
firefox snap does not on the device in question, which has a Snapdragon
X Elite SOC, and the webcam only works with libcamera. This is an Arm
CPU, but we agree it's unlikely the architecture should have an effect
on its own.

Tobias said "what libcamera needs at the very least is raw access to
/dev/media0 and /dev/udmabuf" and "as I understand it that would
normally happen in the pipewire-libcamera plugin which is loaded by
wireplumber", but that "it could also simply be a permission issue"
since "normally udev grants the user permission to the /dev nodes via
uaccess with extended ACLs".

We were curious though whether snaps which use pipewire inside or
outside the snap. In the case of firefox, the firefox snap *does not*
use the pipewire interface: https://github.com/canonical/firefox-
snap/blob/stable/snapcraft.yaml#L47

Instead it seems to ship pipewire internally: Tobias pointed out that
`SPA_PLUGIN_DIR: "$SNAP/usr/lib/$CRAFT_ARCH_TRIPLET/spa-0.2"` is where
libcamera libraries would come from, but the firefox snap does not
install libspa-0.2-libcamera.

So it seems that maybe it's a packaging issue with the firefox snap
failing to bundle the necessary libraries to support libcamera. I'll ask
nteodosio about this, and the question of the pipewire interface.

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https://bugs.launchpad.net/bugs/2121862

Title:
  camera interface breaks libcamera

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