some fonts not showing in Firefox

This is a new 11.1 installation in a netbook (Aspire One) and Firefox is showing a strange behaviour. Some websites don’t show certain fonts, such as the initial table in a Novell Bugzilla page, but all other text looks fine. In some cases no font is shown at all, such as in, just straight lines in place of text. It is not a problem of font size, and I don’t think I am missing any fonts, because I installed the xorg-x11-fonts and xorg-x11-fonts-core packages and a few others, exactly as in other machines that work fine.
Any clues what setting might be wrong?