It seems a while ago Firefox added support for "b=TIAS". Supposedly this can limit the bitrate like "b=AS" can. I'm not really too knowledgable about all this though. I just know that I can limit outgoing bitrate on iOS, Safari, Edge and Chrome, but not firefox.
It would be very nice if Intel WebRTC could detect support for b=TIAS, and convert the maxBitrate publishing option into something compatible for firefox clients, or whatever needs to be done.
Firefox also doesn't support video input scaling, which is annoying, so choosing low resolutions for firefox isn't always an option. Currently I restrict firefox to only 320x240, but it can still use up to 600kbps, usually ~400kbps. Luckily most users of mine don't use firefox, but that might change. If I knew I could restrict firefox's bitrate, I could safely up the quality for everyone without worrying.