You'd be surprised if less-than 99% of Firefox users didn't switch to Chrome to stream television? Am I understanding?
People do this:
https://news.ycombinator.com/item?id=44294402
So they use Firefox 99% of the time and then if they encounter the rare thing that requires DRM they treat it like toxic waste that has to run in an isolated sandbox, which doesn't need to be the same browser they use for anything else.
The only other sensible option is to get out the reversing toolkit and break the DRM.
I think the number of people who care about streaming DRM media probably already used Chrome at the time of the EME stuff being added to Firefox.