Firebug currently (2.0.x) doesn't offer a way to see this relation. One reason for this is that Firebug only displays the CSS rules, which are directly applied to the element or inherited from their parents within the
Style and
Computed side panel according to the
style cascading rules.
Styles defined on
<col> elements are a special case, because they do not follow those rules, because they are styling different elements.
You may
suggest this for the DevTools, though.
Sebastian