I was trying to watch today's charity stream at giantbomb.com/chat with both Abby and Dan's stream, but in Firefox it only showed Abby's stream. However I was also getting the audio from Dan's stream, even though the video player for his stream was not visible.
I also tried loading up a private browsing tab without extensions enabled, and still only got one video player but two audio streams.
Chrome seems to work normally, and shows both streams. Edge sometimes works normally, but sometimes has the same problem that Firefox has.
Firefox 75.0 (64-bit)
Microsoft Edge 44.18362.449.0