Hi!
I was also looking how to change fonts in non-kde applications
that use GTK.
This helped me:
http://www.webservertalk.com/message1152860.html
You have to install those packages:
gtk-qt-engine, gtk-clearlooks and gtk-chtheme.
You can get them here:
http://linux01.gwdg.de/~pbleser/
These packages create a new item "GTK Styles and Fonts" in the ControlCenter of KDE.
There you can set the right options.
After restarting, fonts in firefox and eclipse look fine.