Sometimes some text shows larger than what I expect because the font size in em is based on its parent and the parent on its parent.
I would like to see as a visual cue in Firebug or Web developer instead of going up the tree manually to figure out which parent was the cause of the an expected font size. What I am saying I know what is happening. I just need a tool to view this fact at a glance.
In the style pane for the element, just scroll down till you see the inherited font-size attribute from the parents. It show all inherited attributes, with a strike-through for overridden ones.
This has bothered me on multiple occasions. Unfortunately, firebug does not currently provide the information you are looking for. There is a bug about this issue but it doesn't appear to have had much activity recentyl.