resources/mediawiki.ui/sourcefiles/scss/components/default/_forms.scss gives conflicting CSS for labels. Line 25 specifies
margin: 0; width: 100%
but then line 39 in the same & > div block specifies
width: auto; margin: 0 0 0.2em 0;
The later CSS wins according to Firebug.
Perhaps we can just remove label from the first Sass selector. But then the label isn't styled display: block , and AIUI as a result in Firefox it's squeezed tighter to the input field underneath. I think you get identical effective CSS by adding
display: block; @include box-sizing(border-box);
to the label declaration, but maybe that's overthinking it.