If you are a web designer or if you are at least dealing with fonts time to time, this neat feature should come in handy.
Update: this feature is integrated with Firebug 1.10
Simple feature description:
Highlights the rendered font-family from the list of font-families for a CSS 'font' or 'font-family' property in Firebug's CSS panel.
See all Firebug tips
And couple of explanatory screenshots. The first one shows the Style side panel when FireFontFamily extension is not installed.
The other one shows the same when FireFontFamily is installed.
See that highlighted font, it's the one actually rendered.