ya means yeah, I checked firefox font settings (Tool // options // content tab // font section at the bottom - also the advanced tab for it and that the problem is the entry for "monospace" font seems to randomly select a font each time.
I can see where the problem is but don't know how to fix it. That is, the list of fonts to choose from on the 'monospace' list is only the newly installed ones and don't know how to change it to the list of fonts that came with Windows.