Comment 4 for bug 363666

Do you use any firefox themes? If so - themes can force firefox to use different fonts and sizes than the one you set in gnome-appearance-properties (for ex. MacOSX theme does it).
If this is the case, you may either try to switch back to the default firefox theme or try to tweak your firefox profile using ~/.mozzila/firefox/<your profile name>/chrome/userChrome.css file:

 /*
 * Edit this file and copy it as userChrome.css into your
 * profile-directory/chrome/
 */

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

menu, menuitem {
    font-size: 8pt !important;
    font-family: Sans !important;
}