My Firefox userChrome.css customizations

userChrome.css

/*
 * #region Rounded Tabs
*/

vbox.tab-background {
	border-top-left-radius: 6px;
	border-top-right-radius: 6px;
}

/*
 * #endregion Rounded Tabs
*/

/*
 * #region Hide Blank Spacer in Tabs Row
*/

hbox.titlebar-spacer:nth-child(1) {
    display: none;
}

/*
 * #endregion Hide Blank Spacer in Tabs Row
*/

/*
 * #region Dark Context Menu
 */
 menupopup {
	-moz-appearance: none !important;
	padding: 8px 2px 8px 2px !important;
	background-color: #2B2B2B !important;
	border: 1px #A0A0A0 solid !important;
	min-width: 280px !important;
}
menuseparator {
	-moz-appearance: none !important;
	border: none !important;
	padding: .5px 0px .5px 0px !important;
	margin: 4px 8px 4px 8px !important;
	background-color: #808080 !important;
}
menuitem, menu {
	-moz-appearance: none !important;
	color: white !important;
	padding: 5px 0px 5px 0px !important;
}
menuitem[disabled="true"], menuitem[disabled="true"] > .menu-text {
	color: gray !important;
}
menuitem:hover, menu:hover, menu[open="true"], menuitem[_moz-menuactive="true"]	 {
	-moz-appearance: none !important;
	padding: 5px 0px 5px 0px !important;
	background-color: #414141 !important;
}
menugroup {
	padding: 0px !important;
	background-color: transparent !important;
}
menuitem > .menu-text, menuitem > .menu-iconic-text {
	color: white !important;
}
menuitem[type="checkbox"] {
	padding-left: 8px !important;
	padding-top: 6.5px !important;
	padding-bottom: 6.5px !important;
}
menuitem[type="radio"] {
	padding-left: 8px !important;
	padding-top: 6.5px !important;
	padding-bottom: 6.5px !important;
}
menuitem[type="checkbox"] > .menu-iconic-text, menuitem[type="radio"] > .menu-iconic-text {
	padding-left: 5px !important;
}
menuitem[checked="true"][type="checkbox"] > .menu-iconic-left {
	padding: 7px 0px 0px 0px !important;
	border: solid white;
	border-width: 0 0 2px 2px;
	transform: rotate(-45deg) scale(.75);
	margin-bottom: 4px !important;
}
menuitem[checked="true"][type="radio"] > .menu-iconic-left {
	padding: 1px !important;
	border: solid white;
	height: 17px !important;
	border-radius: 100%;
	background: white;
	transform: scale(.5); 
}
menucaption {
	background-color: transparent !important;
	color: white !important;
}
#ContentSelectDropdown > menupopup {
	background-image: none !important;
}
#ContentSelectDropdown {
	padding: 0px !important;
}
menulist {
	-moz-appearance: none !important;
	padding: 4px 4px 4px 4px !important;
	border: 2px solid #666666 !important;
	background-color: transparent !important;
}
menulist:hover {
	padding: 4px 4px 4px 4px !important;
	border: 2px solid #999999 !important;
	background-color: transparent !important;
}
menulist[open="true"]  {
	border: 2px solid #666666 !important;
	background-color: #333333 !important;
}
#context-back > .menu-iconic-left, #context-forward > .menu-iconic-left, #context-reload > .menu-iconic-left, #context-stop > .menu-iconic-left, #context-bookmarkpage > .menu-iconic-left {
	padding: 0px !important;
}
.menu-iconic-left {
	-moz-appearance: none !important;
	padding: 0px 5px 0px 9px !important;
}
tooltip {
	-moz-appearance: none !important;
	color: white !important;
	background-color: #2B2B2B !important;
	border: 1px solid #767676 !important;
	padding: 6.5px !important;
}
 /*
  * #endregion Dark Context Menu
 */

/*
 * #region Hide Selected Tab Blue Border
*/
.tab-line[selected="true"] {
    opacity: 0 !important;
}
/*
 * #endregion Hide Selected Tab Blue Border
*/

userContent.css

* {
    scrollbar-color: #646a82;
    scrollbar-width: thin;
}

Credit to: https://github.com/Timvde/UserChrome-Tweaks