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Current Set conformance check forces polyfill for Set, but will keep original Array.from in Firefox 32-36. These versions use set["@@iterator"] instead of set[Symbol.iterator] which leads to Array.from(new Set("a")) to be [], although native implementation would return correct result.

I don't know whether

1. Whether it is a concern at all (I think it is even that Firefox 32-36 quite rare they have better feature set than IE11 and probably shouldn't have broken experience)
2. We should expose @@iterator in Set polyfill
3. We should lookup @@iterator in Array.from
4. Other options I haven't thought of

I assume that this can affect other collections and methods that expect iterator.

I would expect the polyfill for Set to pull-in Symbol.iterator since it has 'require es6/symbol';. But, Firefox < 36 does not support Symbol.iterator.

What language level output are you using? That version of Firefox may not support it.

I think language_out it is ECMASCRIPT5