--- a/devtools/client/debugger/test/mochitest/addon-source/browser_dbg_addon3/lib/main.js
+++ b/devtools/client/debugger/test/mochitest/addon-source/browser_dbg_addon3/lib/main.js
@@ -1,13 +1,20 @@
 var { Cc, Ci } = require("chrome");
-var { once } = require("sdk/system/events");
+var observerService = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
 
-var observerService = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
 var observer = {
   observe: function () {
     debugger;
   }
 };
 
-once("sdk:loader:destroy", () => observerService.removeObserver(observer, "debuggerAttached"));
+observerService.addObserver(observer, "debuggerAttached");
 
-observerService.addObserver(observer, "debuggerAttached");
+var sdkLoaderDestroyObserver = {
+  observe: function () {
+    // Remove all observers on sdk:loader:destroy
+    observerService.removeObserver(observer, "debuggerAttached");
+    observerService.removeObserver(sdkLoaderDestroyObserver, "sdk:loader:destroy");
+  }
+};
+
+observerService.addObserver(sdkLoaderDestroyObserver, "sdk:loader:destroy");