Check that #18900 is still fixed
In https://bugzilla.mozilla.org/show_bug.cgi?id=1434666 we checked in a patch that is supposed to fix the problem seen in #18900 (moved). But we should run at least a manual regression test to see that this indeed fixed the bug, and we don't need our #18900 (moved) patch any more for TBB/ESR60.