Iceweasel Menu Font Settings

Add these lines to userChrome.css in ~/.mozilla/firefox/xxxxxxxx.default/chrome

/* Global UI font */
* { font-size: 10pt !important;
font-family: Ubuntu !important;
}

Just replace the font size and the font family with your preferences.

http://kb.mozillazine.org/Pane_and_menu_fonts#Global_UI_font