Supporting Firefox in web apps

To be fair, it's not completely trivial to make a web app compatible with Firefox and Safari.

I've been maintaining a videoconference system <https://galene.org>, and for ideological reasons I'm trying to keep it compatible with Firefox. I've had to work around a number of issues in Firefox's implementation of WebRTC; off the top of my head:

- Simulcast is broken with VP9 <https://bugzilla.mozilla.org/show_bug.cgi?id=1633876>
- Firefox fails to negotiate a trivial simulcast envelope <https://stackoverflow.com/questions/71550933/limiting-web...>
- the `sendEncodings` field is ignored in `sendTransceiver`, you need to manually hack things with `setParameters`;
- the Picture-in-Picture (PiP) API is not implemented.

Now perhaps that's specific to WebRTC, but some of these issues have been reported years ago, and there's still no fix.

I've also had a number of issues with Chrome (incorrectly implemented data channels, for example) and with Safari (overly restrictive autoplay, no filter property on Context2D), but I've found Firefox to be more painful to support than the other two.