The Flexbox ED's "Example 1" sample-rendering doesn't display correctly in Firefox right now -- its two "BUY NOW" buttons render to the left of their containers, and the two buttons are stacked directly on top of each other. (so it looks like there's only one button)

The buttons are absolutely-positioned, and the intended containing block is a "display:table-cell; position:relative" element (with class=computer-example). However, Firefox doesn't honor "position:relative" on table-cells -- so the container doesn't actually form an abspos containing block.

This is tracked in Mozilla's bug-tracker at https://bugzilla.mozilla.org/show_bug.cgi?id=35168 -- as noted in a comment there, the CSS2.1 Spec explicitly leaves this behavior undefined:
"The effect of 'position:relative' on table-row-group, table-header-group,
table-footer-group, table-row, table-column-group, table-column, table-cell,
and table-caption elements is undefined."
http://www.w3.org/TR/2006/WD-CSS21-20060411/visuren.html#choose-position

So this sample-rendering is relying on explicitly-undefined-in-CSS behavior.

If the sample-rendering could be fixed to not rely on this undefined behavior (e.g. by adding a "display:block; position:relative" wrapper inside of the table-cell), that would be awesome.