Don't forget ES6 fat arrows! That and right-dockable dev tools excites me more than they probably should.
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Rel...
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Rel...
The upcoming versions address this issue (in the end, it's all just CSS; for example: [0]).
[0] http://dxr.mozilla.org/mozilla-central/source/browser/themes...