firefox/styles/action-buttons-restyle.css

/* Enable Page Action Button */
#urlbar #page-action-buttons #pageActionButton { visibility: unset; }

/* Hide Hover Zoom+ button */
#urlbar #page-action-buttons #pageAction-urlbar-_92e6fe1c-6e1d-44e1-8bc6-d309e59406af_ { display: none; }

/* Hide Multi-Account Containers button */
#urlbar #page-action-buttons #pageAction-urlbar-_testpilot-containers { display: none; }

/* Hide RES button */
#urlbar #page-action-buttons #pageAction-urlbar-jid1-xufzosoflzsoxg_jetpack { display: none; }

/* Hide reader view button when not active */
#urlbar #page-action-buttons #reader-mode-button:not([readeractive="true"]) { display: none; }

/* Hide default multifoxContainer label */
#urlbar #page-action-buttons #userContext-label[value="Default"] { display: none; }

/* Set multifoxContainer button color to default */
#urlbar #page-action-buttons #userContext-label[value="Default"] + #userContext-indicator,
#mainPopupSet menuitem.identity-icon-fingerprint[label="Default"] .menu-iconic-icon { fill: currentColor; }

/* Set favorites icon color to default */
#urlbar #page-action-buttons #star-button[starred] { fill: currentColor; }