Problem with superscript/subscript alignment on Firefox 26, MathML, HTML/CSS

Created by: Klortho

Here's what I see on Firefox. Note that the problem only manifests when the preceding item "B" has a superscript:

On Chrome it looks fine:

Here's the markup:

<p>With preceding superscript:</p>
<math id="M2" display='block' overflow="scroll">
  <mrow>
    <mi>L</mi>
    <mo>=</mo>
    <msup>
      <mi>B</mi>
      <mi>l</mi>
    </msup>
    <msubsup>
      <mo>&#x2016;</mo>
      <mn>2</mn>
      <mn>2</mn>
    </msubsup>
  </mrow>
</math>

<p>Without preceding superscript:</p>
<math id="M2.wo" display='block' overflow="scroll">
  <mrow>
    <mi>L</mi>
    <mo>=</mo>
    <mi>B</mi>
    <msubsup>
      <mo>&#x2016;</mo>
      <mn>2</mn>
      <mn>2</mn>
    </msubsup>
  </mrow>
</math>