Well, here you are standing on principles. :)
You wanted to watch Youtube vids, so you run Google Chrome, which has even more liberal implementation of this DRM.
You didn't boycott Youtube.
So, this is why Firefox is implementing it. They no longer have the leverage. Google Chrome is bundled with Flash, with Adobe Acrobat, with Oracle Java. It is pushed on every google website people interact with - Search, Plus, Docs, Youtube, Translate. There's the google app store, ChromeOS, Android...
I doubt Brendan would have held out against this either. Firefox' choice is to accede to its users, or become even more marginalised.
I'm glad they are using their limited remaining leverage to try and at least ensure user privacy and security and offer something that is cross-platform, with an open source auditable wrapper and actually works under Linux.