Firefox for Linux always seems to ship with the crappiest possible font configuration. Why? This is what the fonts look like by default on a fresh install of Firefox 1.5, and also in Ubuntu's unreleased "Dapper" distribution's current 1.5 package.
Now, here is the same text rendered using fonts that are packaged with the distribution, and in fact, almost all Linux distributions these days:
"FreeMono", "FreeSans", and "monospace". Was that really so hard?