Because what people really want is a browser that can't use Netflix or Spotify...
Firefox would no longer exist today if they hadn't included DRM. Ideological purity is fun and all, but it's perhaps a good idea to occasionally recognize reality.
So in exchange for their principles, they got to keep 3 whole percent of the market? That's a victory?
I think that's a poor argument. However, I think the stronger argument is that in this case it's actually relatively okay. Like, it'd be a better world if DRM didn't exist, but given that they lack the market power to do anything about that, EME actually seems like the least bad option:
* It's sandboxed.
* It's optional and doesn't run by default.
* Firefox prompts the user and asks if they want to run the DRM.
In fairness, I understand that there are different views on this; I stop one tiny half-step shy of the GNU/FSF position, in that I would argue that people should have complete control of their machines, but that that includes the right to run software that doesn't respect their right to control the machine.