Firefox 3 seems to render Helvetica fonts the wrong way. The solution is to install the right font. Just download Helvetica Neue and install it. (copy to windows/fonts directory).
The font can be found here: http://cinesur.agenciasopa.es/fuentes/
(ps: If you don’t have helvetica neue but any other helvetica, firefox uses that font. I my case it used helvetica neue condensed bolt when it could not find helvetica neue, resulting in almost unreadable fonts).