My personal userChrome.css

userChrome.css

@namespace xul url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

/* Hide disabled page actions: 
   from <https://www.reddit.com/r/firefox/comments/7usqtm/about_extensions_in_address_bar/dtog5qh/> */
#pageActionPanelMainView > .panel-subview-body > toolbarbutton[disabled="true"] {
	display: none !important;
}

/* Hide extraneous page action toolbar separators */
#pageAction-panel-builtInSeparator ~ [disabled="true"] ~ toolbarseparator {
	display: none;
}
#pageAction-panel-builtInSeparator ~ :not([disabled="true"]) ~ toolbarseparator {
	display: inherit;
} 

/* Hide (windowed) Min/Max/Close Buttons: 
   based on <https://support.mozilla.org/en-US/questions/950311> */
#main-window { -moz-appearance: none !important; }
.titlebar-buttonbox-container { display: none !important; }

/* Customize */
:root { --my-focus: #ff9; }
:root :focus:not(:active) {
	--arrowpanel-dimmed: var(--my-focus);
	--arrowpanel-dimmed-further: var(--my-focus);
	--arrowpanel-dimmed-even-further: var(--my-focus);
}

userContent.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

/* Hide old news/annoying notices */
#private-browsing-notice { visibility: collapse; }
#legacy-extensions-notice { visibility: collapse; }

/* Never forget... */
#category-legacy { display: -moz-box; }

/* Extensions tab/section should only show *installed* addons */
*|recommended-extensions-section { display: none; }