browser.xhtml was restructured between firefox 69 and 71, and hints were
not displaying any more (they still worked, adding to the confusion). It
turns out that moving the hints-containert before #navigator-toolbox is
sufficient. But insertBefore()ing that element breaks Waterfox 56, as
this element does not exist there (which is a bit strange, as the docs
say, insertBefore should degrade gracefully to appendChild in such a
case). However, we can just insert it as the first element below
the #main-window element, which works a treat.