定例で
MozillaフォルダのChromeフォルダ内にuserChrome.cssを作って

tab, tab *{
font-family: "Lucida Grande" !important;
font-size: 10px !important;
text-align: center !important;
}
*{
font-family: "Lucida Grande" !important;
}
menu{
font-size:14px !important;
font-family: "Lucida Grande" !important;
}
box , menubutton ,menubar{
font-size:12px !important;
font-family: "Lucida Grande" !important;
}
textfield, textfield-input, .textfield-textarea, textfield.plain
{
font-size:12px !important;
font-family: "Lucida Grande" !important;
}
てな、もんでしょか。