@namespace url(http://www.w3.org/1999/xhtml);
#mainpage {
visibility: visible !important;
}
You can also use @-moz-document rule to specify which pages the style is going to be used on. More info here.
@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document regexp("^https://10\.8\..*") {
#mainpage {
visibility: visible !important;
}
}