Checkbox to spec (bug 944430)

FF18 (legacy Flexbox)
![mozilla_firefox](https://f.cloud.github.com/assets/1514/1641648/26f893d0-585d-11e3-9a35-d3e0f2b5234c.png)

FF25 (current flexbox)
![mozilla_firefox](https://f.cloud.github.com/assets/1514/1641647/1d4f0bca-585d-11e3-8575-ed1ef7c3eb9b.png)