Element Call fails to pick up OBS Virtual Camera from Firefox

### Steps to reproduce

1. Be on Linux, without Pipewire, such that a single application must exclusively use a camera source.
2. Plug in a USB camera.
3. Open OBS and start a virtual camera, using the USB camera as one of the sources.
4. Try to use the virtual camera as your camera in Firefox on Element Call.

This works in Chrom(ium), but not Firefox.

### Outcome

#### What did you expect?

To be able to view the feed of my OBS Virtual Camera during a video call.

#### What happened instead?

The virtual camera feed is detected by Firefox and supplied as the camera to Element Call. Element Call actually shows me my OBS Virtual Camera feed in the call preview screen. However, upon opening settings, the camera feed freezes. And upon joining a call, my camera feed never loads.

In settings, the list of cameras is empty:


Tried on both stable and call.element.dev.

### Operating system

NixOS Linux

### Browser information

Firefox 138.0 (64-bit)

### URL for webapp

call.element.dev

### Will you send logs?

Yes