wget -P ~ ftp://ftp.mozilla.org/pub/firefox/releases/3.0b3/
linux-i686/en-US/firefox-3.0b3.tar.bz2 && tar xjf ~/
firefox-3.0b3.tar.bz2 -C ~
Run your Firefox 3 by
~/firefox/firefoxor double click the shell command in your KDE Dolphin.
Once I got Firefox 3 running I could then get into
layout.css.dpiand reset it to the default '-1'. Changing this to '0' was supposed to force Firefox to use system font setting for menu and toolbar. The trick didn't work for me.
[Edited: a setting between 100~110 looks good to me]