This play with table-layout:fixed; and applying the width to colgroup or col element. Firefox only recongize the width if it is applied on col element!

On a perfect browser, both tables should look the same

colgroup
  300 px width is applied to the first colgroup element

Very long? | # | $

col
  Each colgroup has an additional col element. The first col element is applied the 300 px width

Very long? | # | $