Comment by CivBase

I subscribed to SlingTV a little over a year ago and it did not support Firefox, even with all the DRM enabled. Although that's a problem I blame on SlingTV, not Firefox. It was a known issue which they refused to address. I've since ended my subscription with them.

IIRC, there was also a time when Netflix did not support its highest streaming quality on Firefox. I'm not sure if that's still the case since I also ended my Netflix subscription.

Otherwise I cannot think of any major site which is not supported on Firefox. Outside my employer's fragile intranet, I can't think of any sites which do not support Firefox.