HOWTO: Change menu font of Firefox

1) Create this file:
/etc/X11/xinit/.Xresources

Make perms: root root 644

2) Put this in it:
Xft.dpi: 180

3) Reboot or Restart X for it to take effect.

4) Adjust the dpi value in the above file until the menus look good in your Gtk applications like The Gimp, Firefox, etc., (use multiples of 6).

5) Now adjust your KDE fonts:
KDE Control Center > Appearance & Themes > Fonts > Adjust All Fonts > Tick the size check box > choose the size you want.

Done.

This is exactly what the "Force fonts DPI" select box in KDE Control Center does, except they only give you three choices Disabled, 96 DPI, or 120 DPI.

You'll also notice that this adjusts the fonts in the right frame of KDE Control Center when you choose a directory in the left frame.

For more information see man X under the RESOURCES header and man xrdb.

If you perfer to set it per user, instead of system wide, then create the file here:

~/.Xresources