This works great. Combine it with some tweaks to userChrome.css:

/* Enlarge the icon to 32 pixels: */
#_89772a9a-380f-49b2-bb48-582796985bbe_-browser-action { --toolbarbutton-inner-padding: 0px }
#_89772a9a-380f-49b2-bb48-582796985bbe_-browser-action .toolbarbutton-icon{ width: 32px !important; height: 32px !important; }

/* Remove the hamburger menu: */
#PanelUI-menu-button { display: none !important; }