Fonts dialogue in Firefox, what does that horizontal line indicate?

[Chris G]

When you go to the font selection dialogue in Firefox:-           
          
    Edit - Preferences - Content - Fonts & Colours/Advanced
          
There's a horizontal line in the list of fonts you get to select
when you click on the Serif, Sans-serif or Monospace selection bar.
Does anyone know what this horizontal line indicates?
                                                                 
I originally thought the ones below the line might be the monospace 
fonts but that's obviously not right on inspection.  Is it possibly
that the ones below the line are bit-map fonts?
          
It would actually make selecting a font much easier if there was some
sort of indication against each font to tell you whether it's a
monospace or proportional font.  Some are obvious but not all.

          
Also, a final question, there are fonts one can select called
'monospace', 'serif' and 'sans-serif'.  Presumably if you use these
then what you get is the system default which is defined somewhere in
the build/distribution, there aren't actual font files with these
names.  Have I got this right?                                   

-- 
Chris Green