This was exactly the explanation from Mozilla side and it makes sense. The reason for the decision was people telling "my internet stopped working" (white pages) once they've disabled JS by accident.
Too many people mistakenly disable JavaScript every time a Java vulnerability hits the news, and there was a period not too long ago when a new Java vuln seemed to appear every other week. It was around this time that the "disable JavaScript" option disappeared from Firefox.
It's an interesting thought: should we be dumbing down everything or just engaging in a bit of education instead and helping people understand what JavaScript does and its use on web pages?
The manual for my car mentions things that must be done for maintenance. They don't dumb it down. Meanwhile in computer land we hide all options and dumb everything down to "protect" the user.
There is a difference between being easy to use and molly-coddling. It is a difficult balance to get it seems!
> It's an interesting thought: should we be dumbing down everything
This is not a case of "dumbing down everything", the ability to remove javascript is still there, what is not there is one (of formerly 3) often inadvertently used method for it.
> The manual for my car mentions things that must be done for maintenance.
Disabling javascript is not maintenance, it's a significant structural change in the browser's abilities, like removing the car's suspension. And it makes no sense to put that as a switch on the car's dash.
If you want something used by millions of people, you have to make it as simple as possible. Insisting that people learn about the invisible internals is more likely to put them off using it than to result in them learning.
If you want to continue that, you could consider the manual to be about:config. No one is stopping you from reading the manual or going to about:config, but it's not required.