I think this regression was introduced by https://bugzilla.mozilla.org/show_bug.cgi?id=1499618
Bug 1647078 Comment 17 Edit History
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I think this regression was introduced by https://bugzilla.mozilla.org/show_bug.cgi?id=1499618 which in turn seemed to be a regression from https://bugzilla.mozilla.org/show_bug.cgi?id=1406903 which is "cannot view files using content:// scheme"