tweak-contextmenu.css

/* @replete's Firefox userChrome
 * Context menu tweaks
 * https://github.com/replete/firefox-userchrome
 */


/* Email Image... */
#context-sendimage, 
/* Set Image as Desktop Background... (and preceding separator) */
#context-sep-setbackground, #context-setDesktopBackground
{
    display: none !important;
}