firefox-gnome-theme/theme/matching-autocomplete-width.css

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

/* Set popover size and position */
#urlbar-results {
	--box-width: calc(100vw - var(--item-padding-start) - var(--item-padding-end) + 8px);
	--fixed-widths: calc((6px + 16px + 6px) + (16px + 9px) + 11px);
	border-radius: 9px;
	margin-left: 0 !important;
	margin-top: 0 !important;	
	max-width: var(--box-width) !important;
}

/* Style as regular popover */
#urlbar-results {
	background: var(--gnome-popover-background) !important;
	border: 1px solid !important;
	border-color: var(--gnome-popover-border-color) !important;
	border-radius: 9px;
	box-shadow: var(--gnome-popover-shadow) !important; /* BUG: Not showing any shadow at all */
	color: var(--gnome-toolbar-color) !important;
	padding: 8px;
}

/* Results box */
#urlbar-results .urlbarView-body-inner {
	max-width: calc(var(--box-width) - 18px) !important;
}

#urlbarView-results {	
  --item-padding-start: 0 !important;
	--item-padding-end: 0 !important;
	max-height: 35vh !important;
	overflow-x: auto;
}

/* Search panel  */
#urlbar-results .search-panel-one-offs-header, #urlbar-results .search-panel-one-offs {
	background: transparent !important;
	border: 0 !important;
}
#urlbar-results .search-panel-one-offs-header {
	border-top: 1px solid var(--gnome-popover-separator-color) !important;
}
#urlbar-results .search-panel-one-offs {
	--item-padding-start: 0 !important;
	--item-padding-end: 0 !important;
}

/* Search engines buttons */
#urlbar-results .searchbar-engine-one-off-item:not(.dummy) {
	-moz-appearance: none !important;
	--toolbarbutton-active-background: transparent !important;
	--toolbarbutton-hover-background: transparent !important;
	background: var(--gnome-button-background) !important;
	box-shadow: var(--gnome-button-box-shadow);
	border-width: 1px 1px 1px 0;
	border-style: solid;
	border-color: var(--gnome-button-border-color) !important;
	border-bottom-color: var(--gnome-button-border-accent-color) !important;
	padding: 0 2px !important;
	height: 34px !important;
	color: var(--gnome-toolbar-color) !important;
	box-sizing: border-box !important;
}
#urlbar-results .searchbar-engine-one-off-item:hover, #urlbar-results .searchbar-engine-one-off-item[selected] {	
	background: var(--gnome-button-hover-background) !important;
	color: unset !important;
}
#urlbar-results .searchbar-engine-one-off-item:first-child {	
	border-radius: 5px 0 0 5px;
	border-left-width: 1px !important;
}
#urlbar-results .searchbar-engine-one-off-item.last-engine {	
	border-radius: 0 5px 5px 0;
}
#urlbar-results .searchbar-engine-one-off-item.dummy {
	display: none !important;
}
#urlbar-results .searchbar-engine-one-off-item:not(.last-row) {
	box-sizing: border-box !important;
}
#urlbar-anon-search-settings-compact {
	border-left-width: 1px !important;
	border-bottom: 1px solid var(--gnome-button-border-accent-color) !important;
	border-radius: 5px;	
	margin-left: 6px !important;
}

/* Convert search engines icons to symbolic */
#urlbar-results .searchbar-engine-one-off-item:not(.search-setting-button-compact) .button-icon {
	filter: var(--gnome-convert-icon-to-symbolic-hack-filter);
}

/* Firefox 67 */
/* TODO: Remove when FF =< 67 is deprecated or merge 69+ code. */
#PopupAutoCompleteRichResult {
	margin-left: 0 !important;
	margin-top: -7px;
}
#PopupAutoCompleteRichResult {
  background: var(--gnome-popover-background) !important;
	border: 1px solid !important;
	border-color: var(--gnome-popover-border-color) !important;
	border-radius: 9px;
	box-shadow: var(--gnome-popover-shadow) !important; /* BUG: Not showing any shadow at all */
	color: var(--gnome-toolbar-color) !important;
	padding: 8px !important;
}
#PopupAutoCompleteRichResult .autocomplete-richlistbox {
	--box-width: calc(100vw - var(--item-padding-start) - var(--item-padding-end) - 42px);
	max-width: var(--box-width);
}
#PopupAutoCompleteRichResult .autocomplete-richlistitem {
	padding-left: 0 !important;
	padding-right: 5px !important;
	border-inline-end-width: 0 !important;
	max-width: calc(var(--box-width) - 5px);
	display: flex;
	align-items: center;
}
/* Search panel  */
#PopupAutoCompleteRichResult .search-panel-one-offs-header, #PopupAutoCompleteRichResult .search-panel-one-offs {
	background: transparent !important;
	border: 0 !important;
}
#PopupAutoCompleteRichResult .search-panel-one-offs-header {
	border-top: 1px solid var(--gnome-popover-separator-color) !important;
}
#PopupAutoCompleteRichResult .search-panel-one-offs {
	--item-padding-start: 0 !important;
	--item-padding-end: 0 !important;
}
/* Search engines buttons */
#PopupAutoCompleteRichResult .searchbar-engine-one-off-item:not(.dummy) {
	-moz-appearance: none !important;
	--toolbarbutton-active-background: transparent !important;
	--toolbarbutton-hover-background: transparent !important;
	background: var(--gnome-button-background) !important;
	box-shadow: var(--gnome-button-box-shadow);
	border-width: 1px 1px 1px 0;
	border-style: solid;
	border-color: var(--gnome-button-border-color) !important;
	border-bottom-color: var(--gnome-button-border-accent-color) !important;
	padding: 0 2px !important;
	height: 34px !important;
	color: var(--gnome-toolbar-color) !important;
	box-sizing: border-box !important;
}
#PopupAutoCompleteRichResult .searchbar-engine-one-off-item:hover, #PopupAutoCompleteRichResult .searchbar-engine-one-off-item[selected] {	
	background: var(--gnome-button-hover-background) !important;
	color: unset !important;
}
#PopupAutoCompleteRichResult .searchbar-engine-one-off-item:first-child {	
	border-radius: 5px 0 0 5px;
	border-left-width: 1px !important;
}
#PopupAutoCompleteRichResult .searchbar-engine-one-off-item.last-engine {	
	border-radius: 0 5px 5px 0;
}
#PopupAutoCompleteRichResult .searchbar-engine-one-off-item.dummy {
	display: none !important;
}
#PopupAutoCompleteRichResult .searchbar-engine-one-off-item:not(.last-row) {
	box-sizing: border-box !important;
}