Doesn't work in Firefox?

Question

JavaScript

Almost nothing works in firefox, although everything is ok in chrome. See if you can help.
codepen.io/artekha/pen/PGLRGL

Answer 1

Most likely, except for chrome, it will not work anywhere at all, the definition of a class-object through class so far works fine only in chrome. You need to include babel

Answer 2

in this.selects you have an HTML collection, it does not have a forEach method, rehash it like this

Array.prototype.forEach.call(this.selects, (item, i, selects) => {
      item.addEventListener('mousedown', this.showMenu.bind(this));
    });

also this.lists and options in renderItems