Joshua Ellis complained about Firefox's font rendering not looking as good as Safari's (see the examples posted there). The main difference seems to be just that Firefox stops anti-aliasing (bitmap?) fonts below a limit that is slightly higher than Safari's. If you do the same comparison at the next font size up, they both look equally good:
I think this anti-aliasing limit is intentional, as it makes smaller font sizes easier to read, and less blurry. I can't find a way to change the limit in Firefox though, unlike in Safari.