After searching extensively on why these pseudo-elements are not shown by Firebug in the HTML inspector the way they are in the Firefox and Chrome built-in debugger, I came across a number of results indicating that this is indeed possible. But testing on 4 different computers including Windows and Mac OS I still could not find these in the HTML inspector. Is there some setting I am missing?
Right now I am using Firefox 38.0.5 and Firebug 2.0.11 on Mac Yosemite.
Update:
Actually it looks like this feature is still in development: https://code.google.com/p/fbug/issues/detail?id=5785. The screenshots I saw must have been related to that effort.
Firebug displays pseudo-elements within the Style side panel for the selected element, not within the main HTML panel.