Is this text skinny and hard to read? If you're using Firefox under Windows then the reason is that ClearType anti-aliasing isn't enabled. Windows defaults to uses marginal anti-aliasing for all text and Firefox respects this setting. Internet Explorer, however, overwrites the Windows default which is why many pages look better in IE than Firefox.

This can be easily fixed on XP via a control buried under Display > Appearance > Effects > Smooth edges from 'Standard' to 'ClearType'.