firefox-gnome-theme/theme/parts/findbar.css

/* Find bar  */

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

findbar {
    position: relative;
}

.findbar-container {
	padding: 8px !important;
	display: flex;
	justify-content: center;
	position: relative;
	flex-direction: row;
	width: calc(100% - 42px);
}

hbox[anonid="findbar-textbox-wrapper"] {
	position: absolute;
	right: calc(50vw - 36px);
	transform: translate(50%, 0);
	display: flex !important;
}

.findbar-find-status, .found-matches {
	order: -1;
	padding: 7px 0;
}
.findbar-find-status {
	padding-left: 8px;
	flex: 1;
}

.findbar-closebutton {
	margin: 8px 8px 8px 0 !important;
}