firefox/styles/menu-item-restyle.css

/* Invert Norwell icon */
#widget-overflow #widget-overflow-fixed-list #norwell-button .toolbarbutton-icon { filter: invert(); }

/* Set StyloaiX icon to grayscale */
#widget-overflow #widget-overflow-fixed-list #styloaix-button { filter: grayscale(1); }

/* Set User Scripts icon to grayscale */
#appMenu-popup #appMenu-multiView #appMenu-userChromeJS-button .toolbarbutton-icon { filter: contrast(10) grayscale(1) invert(); }