so DRM is OK as long as it's enclosed within a standardised protocol?
EME is bad if you believe DRM would die/be less common if no standard existed.
Tough luck with that though since EME only specifies how to communicate with DRM, and the actual DRM consists of multiple 100% proprietary black boxes that only work on blessed end devices and browsers. And since different browsers have different black boxes, you need to play ball with Apple, Google, Microsoft, and every other unique DRM vendor.
EME is a disgrace that should have never come to pass. Premium media should rather be forced to peddle their own clients entirely separate from browsers if they insist on non-cross-platform black box bullshit.
I'm not against content protection in itself, but EME was 100% marketed on false pretenses because it's just as much of a proprietary plugin system than what came before, with just as terrible cross-platform story too. If instead we had content protection schemes implemented in eg. WebAssembly, then we could actually talk about a cross-platform system that also doesn't immediately close out smaller browser vendors from the market.
Waterboarding is the better option if you believe that torture is going to happen one way or the other.
The Hollywood studios also have the power not to enforce having to use DRM on their licensees.
Firefox was forced by Chrome, and Chrome in turn was probably forced by trying to remain a viable platform instead of Netflix et al. simply providing their own desktop apps. (Though I'm only guessing, if you have direct knowledge of the what and why, please do share.)
The kind of users who are not opposed against DRM already have lots of browsers to choose between. On the other hand, people who are will now feel deeply betrayed by the Mozilla Foundation.
If you're opposed to a browser that can be used by DRM modules, Firefox was already not an option, since it supported NPAPI, and therefore DRM as Flash, Silverlight, etc. If you were using Firefox, you were already accepting that compromise. EME is simply a continuation of the status quo.
Was there an explicit or implicit covenant by Mozilla and those users?
I believe Mozilla's clear stance on DRM in the past (and its clear differentiation against other browser vendors in this respect) is near to such an implicit covenant.