If you have any objection your X11 setup (especially if you are not running Gnome or KDE) it's probably:
Firefox menu font size is too big!
However, that's really not right complaint. If you create your own userChrome.css and install it in correct location, you will notice that rest of applications still won't look good. But, Firefox was biggest problem, so you move on...
But, solution is really simple:
dpavlin@t61p:~$ cat .gtkrc-2.0 gtk-font-name = "Verdana 7"This will change all GTK application's (including FIrefox, OpenOffice.org and others) to sane font size for huge monitors with high DPI settings. It's a single line fix to all your problems!, take a look: