m.htmlsanity: work around FF misbehavior in <figure> printing.

m.htmlsanity: work around FF misbehavior in <figure> printing.

Counterpart to the previous commit. Done only if the figure has a
.m-figure CSS class, as that's where the Firefox bug is triggered due to
the `display: table-caption` -- code figures and console figures are
unaffected.

There's still a bit of potential future work where the figure
description shouldn't have any block-level elements. But that doesn't
trigger any rendering bugs so not so important.