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.