時代遅れのPale Moonに存在意義なんてあるのだろうか
派生ブラウザならCyberfoxとかWaterfox使えばいいわけだし

http://www.odnous.net/es6-test/
Firefox 28: 16/28　　Pale Moon 26.5.0: 16/28
Firefox 31: 17/28
Firefox 38: 23/28
Firefox 40: 24/28　　Pale Moon 27.0.3: 24/28
Firefox 45: 27/28
Firefox 50: 28/28

https://html5test.com/
Firefox 24: 351/555　　Pale Moon 26.5.0: 351/555
Firefox 31: 390/555
Firefox 35: 397/555　　Pale Moon 27.0.3: 398/555
Firefox 38: 420/555
Firefox 45: 444/555
Firefox 50: 466/555

http://css3test.com/
Firefox 28: 44%　　Pale Moon 26.5.0: 44%
Firefox 31: 45%
Firefox 38: 52%　　Pale Moon 27.0.3: 52%
Firefox 45: 62%
Firefox 50: 65%