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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.