Damit das Icon von uBlock in diese Position kommt

/* Abstände für Icons und Text von Erweiterungen im Kontextmenü */ 
@-moz-document url(chrome://browser/content/browser.xhtml) {

/* für alle Erweiterungen Icon verlagert (nach links) */    

#contentAreaContextMenu .menu-iconic > .menu-iconic-left,
#contentAreaContextMenu .menuitem-iconic > .menu-iconic-left {
    margin-left: -28px !important;
}

/* der Text dazu verlagert */

#contentAreaContextMenu .menu-iconic > label.menu-iconic-text,
#contentAreaContextMenu .menuitem-iconic > label.menu-iconic-text {
    margin-left: 5px !important;
}


/*===== uBlock =====*/

#ublock0_raymondhill_net-menuitem-_uBlock0-blockElement.menuitem-iconic hbox.menu-iconic-left image.menu-iconic-icon {
    margin-left: 0px !important;
}

/* Submenü dazu    */

/* Element blockieren */
#contentAreaContextMenu .menu-iconic menupopup menuitem#ublock0_raymondhill_net-menuitem-_uBlock0-blockElement{
    background-image: url("file:///C:/FoxIcons/1127.png") !important;
    background-repeat: no-repeat !important;
    background-position: 8px 4px !important;
    background-size: 15px 15px !important;
}

/* Element im Frame blockieren */
#contentAreaContextMenu .menu-iconic menupopup menuitem#ublock0_raymondhill_net-menuitem-_uBlock0-blockElementInFrame {    
    background-image: url("file:///C:/FoxIcons/1126.png") !important;
    background-repeat: no-repeat !important;
    background-position: 8px 4px !important;
    background-size: 15px 15px !important;
}

/* Quellcode anzeigen */
#contentAreaContextMenu .menu-iconic menupopup menuitem#ublock0_raymondhill_net-menuitem-_uBlock0-viewSource{
    background-image: url("file:///C:/FoxIcons/1252.png") !important;
    background-repeat: no-repeat !important;
    background-position: 10px 4px !important;
    background-size: 15px 15px !important;
}    
}