fix(button): Fix button height in Firefox.

A `<button>` tag with the `.md-button` class added was rendering
at 38px height instead of the spec's 36px. This was due to the
`::-moz-focus-inner` pseudo-class having a default border greater
than 0.

Fixes #3291.

src/components/button/button.scss

@@ -19,6 +19,11 @@ $icon-button-height: rem(4.800) !default;
 $icon-button-width: rem(4.800) !default;
 $icon-button-margin: rem(0.600) !default;
 
+// Fix issue causing buttons in Firefox to be 2px bigger than they should
+button.md-button::-moz-focus-inner {
+  border: 0;
+}
+
 .md-button {
   border-radius: $button-border-radius;
   box-sizing: border-box;