firefox/chrome/userChrome.css

/* Remove fullscreen transition animation */
#navigator-toolbox[fullscreenShouldAnimate] { transition: none !important; }

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

/* Enable page action button */
#urlbar #page-action-buttons #pageActionButton { visibility: unset !important; }

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

/* Set reader view button color to default */
#urlbar #page-action-buttons #reader-mode-button[readeractive] > .urlbar-icon { fill: currentColor !important; }

/* 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 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; }

/* Shrink bookmark separators */
#PlacesToolbarItems > toolbarseparator::before {
	filter: opacity(0.3);
	height: 80%;
	margin: auto;
}

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