You're assuming that this will kill FF instantly instead of them finding new ways to fund themselves. Not saying the situation is good, but it's not that bad.
We know how it will impact Firefox. They will be deeply concerned with WEI and extremely opposing it, but will implement it anyway because they are forced to do it.
With most competing browsers and the content industry embracing the W3C EMEWEI specification, Mozilla has little choice but to implement EMEWEI as well so our users can continue to access all content they want to enjoy.
Someone will most likely create a fork to remove this or an option to disable it will probably be baked into about:config. I don't visit many sites that use DRM. When I do visit sites that require it, I'll usually shift to Ungoogled Chromium or Brave.
Another one of my major fears with this change is whether Google will decide to make Chromium closed source and the implications it can have for other chromium based browsers.
Someone will most likely create a fork to remove this or an option to disable it will probably be baked into about:config.
But that's the issue - if WEI passes, EVERY webpage will be able to use DRM. So, just like you have to switch to Chromium for DRMed media content, you will now have to switch for every website that has decided to implement it. So, your bank (because google is pushing it as a security feature), Youtube, Gmail... Just like you are not able to play DRM media, you won't be able to visit DRMed websites without WEI API supported. It's not something you turn off.
I'd be fine ditching youtube and gmail instead of having to use their shitty services with ads that lead to fake software that compromise your computers. Also my bank could try this, I would quickly turn into their most annoying customer also it could get to the point that unless you run a specific OS these pages won't work and it would start being problematic legally for them. But until then I will pray everyday that the EU will take a hammer to WEI and break it's legs.