I am using Firefox with Wayland+PipeWire on Trixie together with a v4l2loopback virtual camera.

The virtual camera is generated from an older Logitech QuickCam Express/Go (lsusb says vendor is 046d and product is 092f):

LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libv4l/v4l2convert.so ffmpeg -f v4l2 -framerate 9 -video_size 352x288 -i /dev/video0 -vf format=yuv420p,fps=30 -c:v rawvideo -pix_fmt yuv420p -f v4l2 /dev/video10

The resulting loopback device advertises:

     Pixel format: YU12
     Resolution:   352x288
     Frame rate:   30 fps

The virtual camera itself appears to work correctly:

- ffplay -f v4l2 /dev/video10 displays live video.
- Chromium can use the virtual camera on both https://webrtc.github.io/samples/src/content/getusermedia/resolution and https://webcamtests.com.
- Firefox successfully uses the virtual camera on https://webcamtests.com/.
- Firefox can enumerate the virtual camera and open it via navigator.mediaDevices.getUserMedia().

For example, after granting camera permission:

     await navigator.mediaDevices.getUserMedia({
       video: {
         deviceId: { exact: vcam.deviceId },
         width: { exact: 352 },
         height: { exact: 288 }
       }
     });

succeeds.

Likewise:

     width: { ideal: 320 }
     height: { ideal: 240 }

also succeeds.

As expected,

     width: { exact: 320 }
     height: { exact: 240 }

fails with

     OverconstrainedError

because the camera only advertises 352x288.

However, on https://webrtc.github.io/samples/src/content/getusermedia/resolution, selecting the virtual camera and clicking "QVGA" produces

     getUserMedia constraints:
     {"video":{"width":{"exact":320},"height":{"exact":240}}}

followed by

     AbortError
     Starting videoinput failed

instead of the OverconstrainedError observed when issuing the equivalent getUserMedia() call directly from the Firefox console.

This suggests either:

- Firefox behaves differently during the request sequence used by the sample than during a direct getUserMedia() call, or
- there is an interaction between Firefox and the sample that results in AbortError instead of the expected OverconstrainedError.

Since Firefox successfully streams from the same virtual camera on webcamtests.com and through direct JavaScript calls, the virtual camera itself does not appear to be the cause.

Steps to reproduce:

1. Create a v4l2loopback device exposing only 352x288 YU12.
2. Feed it using the FFmpeg command above.
3. Open https://webrtc.github.io/samples/src/content/getusermedia/resolution/
4. Select the virtual camera.
5. Click “QVGA”.

Expected result:

The browser either reports OverconstrainedError for the unsupported 320x240 exact constraint or otherwise behaves consistently with direct getUserMedia() calls.

Actual result:

AbortError:
"Starting videoinput failed"

If this is not considered a Debian-specific issue, please forward it upstream to Mozilla (or, if appropriate, to the webrtc/samples project).