[perf] fix sidebar performance problem

xpfe/components/sidebar/resources/sidebarBindings.xml

@@ -31,14 +31,14 @@
 
   <binding id="sidebar-header-box" extends="xul:box">
     <content autostretch="never" valign="middle">
-      <xul:text inherits="value,crop" crop="right" flex="1"/>
+      <xul:text class="sidebar-header-text" inherits="value,crop" crop="right" flex="1"/>
       <xul:box><children/></xul:box>
     </content>
   </binding>  
 
   <binding id="sidebar-header-splitter" extends="xul:splitter">
     <content autostretch="never" valign="middle">
-      <xul:text inherits="value,crop" crop="right" flex="1"/>
+      <xul:text class="sidebar-header-text" inherits="value,crop" crop="right" flex="1"/>
       <xul:box><children/></xul:box>
     </content>
   </binding>