tip - for KDE users.

[S Beam]

just a nice thing to know - 

Firefox uses GTK to draw its menu and toolbar, as of course do Gnome 
apps like Gimp and Gaim... This means when you use KDE Control Center 
to set your fonts to a nice pleasing just-right font, it has no effect 
on the toolbars and menus in the GTK apps. In my case they looked too 
big and goofy next to the pleasingly rigid uniformity of my KDE apps. 
Depending on your level of perfectionism this may drive you nuts. The 
solution (after many many dead ends) was to create a file ~/.gtkrc-2.0 
as follows:

$ cat ~/.gtkrc-2.0
gtk-font-name="Sans 8"

thats it! Adjust the font to taste. No messing around with FF settings 
userChrome.css or xfontsel or anything else is needed.