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See https://bugzilla.mozilla.org/show_bug.cgi?id=120556 You see a typo on your perfectly valid page, save it locally, fix the typo, upload it, and suddenly it's no longer valid.
Why is this a validator problem? Firefox appears to be broken in this regard, very clearly. If the moz developers don't want to fix it, the tool will remain broken. That's too bad, but how is it relevant here?
It's not a validator bug at all indeed. Just looking to bring attention to the matter.