Put mozilla eslint rules back in

[ianb]

@Standard8 is looking at the problems we've encountered.

Commit 2d01ca1 removed the use of the mozilla plugins; undoing that commit should put things back.

[Standard8]

I think I've tracked this down to a module dependency issue - I'm currently working on a patch for https://bugzilla.mozilla.org/show_bug.cgi?id=1395879. Once that has landed, I'll get a new version published, which will hopefully work better.