ES6 introduced a change in Gecko that remained under my radar for a looong time. Testing BlueGriffon 2.0 features, I discovered a feature that was completely borked and I was unable to explain it... Everything in my code looked fine. After some debugging, I nailed it into a codepen. Gecko and Chrome show the behaviour I dislike (window.bar is undefined if bar is a constant) while Safari does not.

I find this, mandatory per ES6 spec, completely ridiculous. It's not understandable from a JS author's perspective and apparently broke "a ton of stuff". This is clearly the kind of things where gurus and purists should have thought of users (JS authors) and did not.

Please TC39, change that. It's ugly and painful.