Event.returnValue is legacy attribute which was introduced by IE. We
released this feature at 63, but it broke an internet application (bug
1478102).
Therefore, we unshipped it temporarily at 64 because the fix of bug
1479964 will fix the intranet application. However, unfortunately, the
fix broke another internet app (bug 1514940).
Therefore, we put off to reship this at least until 66.
--
Masayuki Nakano <masay...@d-toybox.com>
Software Engineer, Mozilla
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