Yes, you could implement DRM through binary plugins before, but there's still a difference between that and explicitly supporting DRM. If you don't see that difference, I'm unable to explain it because to me that is just so absolutely clear.
I understand the difference from a narrow ideological argument, but can you see that the current system is worse for internet freedom than the system they are proposing?
After installing Firefox as soon as you start browsing video sites you are actively prompted to download and install a dangerous, closed source proprietary plugin that has complete access to your local system resources. Firefox has always supported this, and never tried to prevent this from happening.
The new system removes any need for that plugin, instead any proprietary component is reduced to the bare minimum and run in a sandbox.
It just makes no sense to denounce Mozilla for offering an extremely limited video decoding API while ignoring the fact they actively support monstrosities like Flash.