Pion WebRTC v4 doesn't work with sfu-ws on Firefox

[okyanusoz]

I'm not sure where to file this bug, so I'm filing it here.

If you try running the sfu-ws sample here and change it to use Pion WebRTC v4 instead:
import (
//...
"github.com/pion/webrtc/v4"
)

It will work on Chrome, but not Firefox. Specifically, it works with 2 participants but not more for some reason. It worked with v3.

Your environment.

• Version: v4
• Browser: Firefox 129.0.2 on macOS
• Other Information - Report below.

What did you do?

Try using Pion WebRTC v4 with sfu-ws or any other SFU-like app.

What did you expect?

I expected 3 participants to work. Instead, one participant successfully connect, but others just freeze or slow down significantly.

What happened?

Taking a look at about:webrtc-internals shows this:

As you can see, only one participant is able to both send and receive VP8 when we have 3 participants.

A full report from a peer who froze is below:
debug.json

Thanks!