Bugzilla

It looks like, for this JS Error's purposes, "iterable" literally means "has a property called Symbol.iterator, which returns a function that can be called", per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/is_not_iterable .
 
And indeed, this returns `Undefined` in Firefox (rather than a function):
document.fonts.keys()[Symbol.iterator]
...whereas it returns a callable function in Chrome (and if I invoke that function, it returns an Iterator).

Given that our `FontFaceSetIterator` object does have a next() function (and hence behaves like an iterator from that standpoint), maybe we just need something akin to the trivial `[Symbol.iterator]: function() { return this; }` boilerplate on that MDN page, I wonder?

It looks like, for this JS Error's purposes, "iterable" literally means "has a property called Symbol.iterator, which returns a function that can be called", per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/is_not_iterable .
 
And indeed, this returns `Undefined` in Firefox (rather than a function):
```
document.fonts.keys()[Symbol.iterator]
```
...whereas it returns a callable function in Chrome (and if I invoke that function, it returns an Iterator).

Given that our `FontFaceSetIterator` object does have a next() function (and hence behaves like an iterator from that standpoint), maybe we just need something akin to the trivial `[Symbol.iterator]: function() { return this; }` boilerplate on that MDN page, I wonder?