AMScd doesn't look good in Firefox NativeMML mode

Vertical arrows should get lspace and rspace set to 0 to help improve the results.

There still seems to be a bug in FF that prevents arrows from stretching when they are the base of an <mover> or similar element and there are not other elements in the container for the <mover>. E.g.

<math>
  <mover>
    <mo stretchy="true">&#x2192;</mo>
    <mi>aaaaaaaaaa</mi>
  </mover> 
</math>

doesn't stretch, but

<math>
  <mi></mi>
  <mover>
    <mo stretchy="true">&#x2192;</mo>
    <mi>aaaaaaaaaa</mi>
  </mover> 
</math>

does.

Also,

<math>
  <mover>
    <mo minsize="2em" stretchy="true">&#x2192;</mo>
    <mi>aaaaaaaaaa</mi>
  </mover> 
</math>

doesn't work.