Having a choice doesn't magically resolve the issues, it doesn't prevent content providers from making things formerly available unavailable, it doesn't help us (as users) make better more considered decisions rather than clicking yes a lot to get things done, etc. But it does retain the choice to not run this stuff, choice which is not retained in the current closed source alternatives.
The best option might be to sink this development energy into something that will draw users from other browsers, if they can figure out what...
You say Mozilla should figure out what will draw users from other browsers, but you've outlined what will draw users from Mozilla to other browsers. In the end, people want content. The content providers have more control. If 70-80% of the browsers already support the DRM they want to use, the other 20-30% will follow or be left out. Which is exactly what happened here.
All of that said, yes, of course this is something that will serve as a draw for users away from Mozilla. And yes, of course Mozilla might not be able to find something else that can serve as a better (or even comparable) draw that will not compromise their principles at least as much. I'm not casting aspersions at Mozilla and saying "Gee, they're dumb, why don't they just...". I'm saying "It would be better if they had found X" where I'm not at all sure such an X exists.
The problem is not that Mozilla implementing EME removes leverage to force Netflix into abandoning DRM (this won't happen anyway anytime soon), the problem is that once DRM is readily available without a large technological investment and without losing too many users, virtually everyone will use it for virtually everything, simply because there is little to none incentive not to do it, from a publisher's perspective.
I'd rather have a (albeit stupid) DRM executable that only performs encryption/decryption than Flash, Java, or Silverlight, which are the current solutions for this. All three of these offer APIs for doing things other than decrypting DRM content, and these APIs have been proven time and time again to be vulnerable to attack, no matter how much time is spent trying to sandbox them properly.
Really, come on. You should understand this. Firefox isn't like Chromium, where the 'blessed' Google version, Chrome, has dozens of important features sitting behind proprietary black boxes - the Firefox you download off Mozilla's website is something you could literally compile yourself. I've compiled Firefox dozens of times, sometimes with in-development patches for things like gamepad support added in.
Your fear is pure paranoia.
It's not like anybody can force you to watch the last Hollywood stuff.
In the land of PDF you can find things like random federal agencies slapping the protection bits on complete inexplicable things, even though the content is public domain by law. ... just because the knob is there.
The 'you can not use it' opt-out ignores the cost of actually discovering that you should. The Mozilla proposal is better than that because it will tell you. This still doesn't solve all the issues, but its better than you'll find in other popular browsers.