gistfile1.xml

<bindings 	xmlns="http://www.mozilla.org/xbl"
			xmlns:xbl="http://www.mozilla.org/xbl"
			xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
	<binding id="tab-counter" extends="chrome://global/content/bindings/tabbox.xml#tab">
		<content>
			<xul:hbox class="tab-middle box-inherit" xbl:inherits="align,dir,pack,orient,selected" flex="1">
				<xul:image class="tab-icon" xbl:inherits="validate,src=image" role="presentation"/>
				<xul:label class="tab-text" xbl:inherits="value=label,accesskey,crop,disabled" flex="1" role="presentation"/>
				<xul:label class="tab-counter" xbl:inherits="value=_counterLabel" role="presentation"/>
			</xul:hbox>
		</content>
		<implementation>
			<property name="count">
				<setter>
				<![CDATA[
					var v = parseInt(val, 10);
					if(!isNaN(v) && v >= 0) {
						this.setAttribute('count', v);
						this.setAttribute('_counterLabel', '(' + v + ')');
					}
				]]>
				</setter>
				<getter>
					return this.getAttribute('count');
				</getter>
			</property>
			<property name="_counterLabel" onget="return this.getAttribute('_counterLabel');" onset="this.setAttribute('_counterLabel', val);"/>
		</implementation>
	</binding>
</bindings>