Изменяем через userchrome.css :
menuitem{
color: #000 !important;
background: transparent !important;
font-family: verdana !important;
font-weight: normal !important;
font-size: 13px !important;
}
menu{
color:#000 !important;
background:transparent !important;
font-family:verdana !important;
font-weight:normal !important;
font-size:13px !important;
}