It's even more relevant now with all the front end JavaScript frameworks that so many web apps rely on these days. In an age where web sites are now web apps that rely almost totally on JavaScript to function removing this option except for those who really have a good reason is acceptable and probably preferable. I feel like educating users is becoming harder and harder due to how hard we've all pushed for "it just works" type experiences so rather than fight the tide Mozilla is going with it.

Plus, apart from security concerns which can be dealt with other ways, JavaScript engines are now capable of running multiple web apps in many tabs simultaneously without being noticed. Wasn't performance one of the primary reasons for having this option back in the day? Now its not an issue. Going forward we might not see a disable js button and think it just as normal as not seeing an option to disable CSS or even html. It's kind of a non-optional piece of the web now.