Scroll down a bit, the only reason it's alive in bugzilla is because it's intended solely for use in Fennec, the current attempt at "Mobile Mozilla":  <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=422540#c105" rel="nofollow">https://bugzilla.mozilla.org/show_bug.cgi?id=422540#c105</a>