In this previous post, I said I had found a way to adjust the eclipse fonts (that are gtk fonts) in KDE. That was a right solution, but unfortunately, it does no survive logouts from KDE. Each time you should re-run the gnome configuration program.
However, that's an even nicer solution: install the package gtk-qt-engine:
The GTK-Qt Theme Engine is a plugin for GTK that allows GTK applications to use Qt widget styles. Aimed primarily at KDE users, this plugin provides a way to unify the look and feel of the Linux desktop.
this will add the additional menu in the KDE Control Center - Appearance and Themes that will do the trick! Now Eclipse, Firefox and Thunderbird look very nice with the same KDE style.