Added onclose handler to return false. Fixes M13 bug #17019. r=ssu

xpinstall/res/content/xpistatus.xul

@@ -31,6 +31,7 @@
 <window xmlns:html="http://www.w3.org/TR/REC-html40"
         xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
         onload="onLoad()"
+        onclose="return false"
         title="&progress.label;"
 		align = "vertical"
 		class = "dialog"