KOReader: a free electronic-book reader for e-ink devices

The DRM you can enable in Firefox is (or can be?) extremely CPU-intensive, and runs in 1 thread even if you are running multiple streams at the same time (which people like to do to e.g. follow some sports events that happen at the same time). Obviously whenever it tries to use more than 100% of that single CPU core things start to hang & stutter, a problem illegal non-DRM streams don’t suffer from—especially with hardware video decoding.

Many streaming services only allow it for lower resolutions also (requiring hardware DRM for better qualities).