--- a/testing/mochitest/ShutdownLeaksCollector.jsm
+++ b/testing/mochitest/ShutdownLeaksCollector.jsm
@@ -16,18 +16,24 @@ var ContentCollector = {
   init: function() {
       let processType = Cc["@mozilla.org/xre/app-info;1"].getService(Ci.nsIXULRuntime).processType;
       if (processType == Ci.nsIXULRuntime.PROCESS_TYPE_DEFAULT) {
         // In the main process, we handle triggering collections in browser-test.js
         return;
       }
 
     Services.cpmm.addMessageListener("browser-test:collect-request", this);
+    Services.obs.addObserver(this, "outer-window-destroyed");
+    Services.obs.addObserver(this, "outer-window-nuked");
+    Services.obs.addObserver(this, "inner-window-destroyed");
+    Services.obs.addObserver(this, "inner-window-nuked");
   },
 
+  destroyedWindowIDs: new Set(),
+
   receiveMessage: function(aMessage) {
     switch (aMessage.name) {
       case "browser-test:collect-request":
         Services.obs.notifyObservers(null, "memory-pressure", "heap-minimize");
 
         Cu.forceGC();
         Cu.forceCC();
 
@@ -37,28 +43,77 @@ var ContentCollector = {
             // as possible is freed.
             Cu.forceGC();
             Cu.forceCC();
             aCallback();
           });
         };
 
         shutdownCleanup(() => {
-          setTimeout(() => {
-            shutdownCleanup(() => {
-              this.finish();
-            })
-          }, 1000);
+          this.doneDestroyingWindows().then(() => {
+            setTimeout(() => {
+              shutdownCleanup(() => {
+                this.finish();
+              })
+            }, 1000);
+          });
         });
 
         break;
     }
   },
 
+  observe(subject, topic, data) {
+    // Subject for outer-window-destroyed and outer-window-nuked
+    // should be the outerWindowID wrapped in an
+    // nsISupportsPRUint64
+    let id = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
+
+    switch (topic) {
+      case "inner-window-destroyed":
+        // Fall-through
+      case "outer-window-destroyed": {
+        this.onWindowDestroyed(id);
+        break;
+      }
+      case "inner-window-nuked":
+        // Fall-through
+      case "outer-window-nuked": {
+        this.onWindowNuked(id);
+        break;
+      }
+    }
+  },
+
+  onWindowDestroyed(windowID) {
+    this.destroyedWindowIDs.add(windowID);
+  },
+
+  onWindowNuked(windowID) {
+    this.destroyedWindowIDs.delete(windowID);
+    if (this._resolver) {
+      this._resolver();
+    }
+  },
+
+  doneDestroyingWindows() {
+    if (!this.destroyedWindowIDs.size) {
+      return Promise.resolve();
+    } else {
+      return new Promise(resolve => {
+        this._resolver = resole;
+      });
+    }
+  },
+
   finish() {
     let pid = Cc["@mozilla.org/xre/app-info;1"].getService(Ci.nsIXULRuntime).processID;
     dump("Completed ShutdownLeaks collections in process " + pid + "\n");
 
     Services.cpmm.removeMessageListener("browser-test:collect-request", this);
+    Services.obs.removeObserver(this, "outer-window-destroyed");
+    Services.obs.removeObserver(this, "outer-window-nuked");
+    Services.obs.removeObserver(this, "inner-window-destroyed");
+    Services.obs.removeObserver(this, "inner-window-nuked");
   },
 
 };
 ContentCollector.init();