So what was their option? Refusing to implement DRM (as opt-in) would mean that the ignorant user wouldn't be able to see Youtube videos anymore. The result would be that these users would move over to Chrome, Safari or Edge. There was no alternative.
Mozilla is very upfront about the fact that it goes against their principles. https://hacks.mozilla.org/2014/05/reconciling-mozillas-missi... Anyway it's opt-in, it's easy to uninstall the one CDM it comes with, and it's fairly easy to stop it from even asking about downloading CDMs. The EME-free build just does all of that for you up front.
Yes, there is an option, and it has been linked repeatedly in these threads. The irony being, of course, that if Firefox didn't yield to the other browser vendors in implementing EME they wouldn't have the marketshare to keep development going, which means there would be zero browsers rather than one that make it trivial to forgo DRM.