(I don't know if there's a better Phabricator project forthis.)

In Firefox, no icons appear in the various http://wikimedia-ui.wmflabs.org/ examples. There are no icons in "Add an icon" example, nor the down pointer in "Button + Dropdown":

<wiki-icon icon="caret-down">
  <svg class="icon wikiglyph-caret-down" viewBox="0 0 512 512">
      <path d="m256 427l190-190c-17-17-43-17-60 0l-130 130-130-128c-17-17-43-17-60 0z"></path>
   </svg>
</wiki-icon>

(good!), but in Firefox 42 it's just

<wiki-icon icon="caret-down"></wiki-icon>

with no embedded SVG. Perhaps the missing SVG is why the size of the div for the caret down is smaller than the button to its left.