The problem with MacUpdate--and I've never experienced the problem myself, but I've seen that screenshot enough times to know that it's real--is that MU is declaring Geneva first in all its styles.
Guess what? Geneva doesn't have a bold version (try it in TextEdit and see!), so apps have to try and fake it. For whatever reason, Gecko fake bold isn't as good as WebKit fake bold.
Fortunately, not only does MU have a dumb set of fonts, they have a really dumb set of fonts. So a quick
Code:
@-moz-document domain(mail.google.com) {
* {font-family: Arial, Helvetica, Verdana, 'Lucida Grande', sans-serif !important; }
}
(or whatever font you'd prefer that actually has a bold weight; you only really need one) should fix it. You may want to re-override the two styles where they specify some font other than Geneva first. Note that I haven't tried this, but it should work. You can add "similar" code to other sites which insist on using Geneva (or other fonts without bold versions) "bold".
Another solution is to disable Geneva, as long as it's not a font that the OS requires.
Finally, some good news: I looked at this in the Fx build with the MLTE patches, and it looked good there, sharper than in normal Fx, so you might not have to wait for Cairo--there's an intention to land the MLTE patch, if it gets a positive review, on the 1.8 branch.