--- a/dom/tests/mochitest/webapps/test_bug_765063.xul
+++ b/dom/tests/mochitest/webapps/test_bug_765063.xul
@@ -16,16 +16,17 @@
   <script type="application/javascript" src="jshelper.js"/>
 
 <script> 
 SimpleTest.waitForExplicitFinish();
 
 // Observe app installation and confirm that the install origin didn't change.
 var observer = {
   observe: function observe(subject, topic, data) {
+    Services.obs.removeObserver(observer, "webapps-sync-install");
     ok(JSON.parse(data).installOrigin == "http://www.example.com",
        "the install origin didn't change");
     tearDown();
     SimpleTest.finish();
   }
 };
 
 Components.utils.import("resource://gre/modules/Services.jsm");