The difference between the fonts is the difference between a slant of `o' (which looks OK) and one of `i' (which looks wobbly). I suppose Mozilla doesn't care which it uses, so it depends on which one has been cached by my X server. Running xfontsel causes the `i' version to get cached (either because it comes earlier in the alphabet or because it causes the names of all fonts to be evaluated).
I think that's a plausible explanation, anyway. Unfortunately I don't know how to un-cache the font so I'm probably stuck with the ugly version for the time being.
In other news, my browser suddenly came up with much darker colours and smaller fonts in the toolbars the other day, which I eventually traced to the fact that some application had written a .gtkrc file in my directory without telling me. At least now I know about this file I can (in theory) customise Mozilla a bit. I'm experimenting with a smaller font so I can see a bit more in the Location bar. As long as I stick with adobe-helvetica rather than linotype-helvetica it remains readable (I also tried helvetica-narrow but it seemed to ignore the `narrow' bit). It does however also affect some text-entry boxes and buttons on ordinary web pages.