Firefox 56 has introduced a regression on CSS multi-column layouts due to a performance enhancement. One report shows a two-column layout balanced unevenly. Another bug has been found ironically on this FxSiteCompat.com where
::before pseudo-elements randomly disappear within a multi-column layout if combined with
float:left. Troublingly, the behaviour is capricious in both cases. Mozilla developers will look into the issues shortly.
Update: This bug has been fixed with Firefox 70.