[janvidar]

It is not entirely irrelevant.

Given that there are more developers familiar with HTML development, it may lower the barrier to developing plug-ins, or providing patches.

[emidln]

I was poking at the non-standard portion. Lowering barrier to development of plugins, patches, etc is fine, but being standard doesn't help or hinder that. Popularity and ease of introduction help that. Javascript (that is, the dialect of ECMAScript that is implemented by Firefox) itself is non-standard and Mozilla isn't throwing that one out for exclusively ES2015.

[pcwalton]

Actually, tons of nonstandard SpiderMonkey features have been removed. Sharp literals were axed, E4X was removed, "let" is being changed to the ES6 behavior, and so on.

[gsnedders]

FWIW, there are real intentions to standardise all the JS extensions or drop them.