--- a/browser/experiments/Experiments.jsm
+++ b/browser/experiments/Experiments.jsm
@@ -322,16 +322,24 @@ Experiments.Policy.prototype = {
     });
   },
 
   telemetryPayload: function () {
     return TelemetryPing.getPayload();
   },
 };
 
+function AlreadyShutdownError(message="already shut down") {
+  this.name = "AlreadyShutdownError";
+  this.message = message;
+}
+
+AlreadyShutdownError.prototype = new Error();
+AlreadyShutdownError.prototype.constructor = AlreadyShutdownError;
+
 /**
  * Manages the experiments and provides an interface to control them.
  */
 
 Experiments.Experiments = function (policy=new Experiments.Policy()) {
   this._log = Log.repository.getLoggerWithMessagePrefix(
     "Browser.Experiments.Experiments",
     "Experiments #" + gExperimentsCounter++ + "::");
@@ -429,17 +437,21 @@ Experiments.Experiments.prototype = {
 
       if (this._timer) {
         this._timer.clear();
       }
     }
 
     this._shutdown = true;
     if (this._mainTask) {
-      yield this._mainTask;
+      try {
+        yield this._mainTask;
+      } catch (e if e instanceof AlreadyShutdownError) {
+        // We error out of tasks after shutdown via that exception.
+      }
     }
 
     this._log.info("Completed uninitialization.");
   }),
 
   _registerWithAddonManager: function () {
     this._log.trace("Registering instance with Addon Manager.");
 
@@ -452,17 +464,17 @@ Experiments.Experiments.prototype = {
     AddonManager.removeAddonListener(this);
   },
 
   /**
    * Throws an exception if we've already shut down.
    */
   _checkForShutdown: function() {
     if (this._shutdown) {
-      throw Error("uninit() already called");
+      throw new AlreadyShutdownError("uninit() already called");
     }
   },
 
   /**
    * Whether the experiments feature is enabled.
    */
   get enabled() {
     return gExperimentsEnabled;

--- a/toolkit/mozapps/extensions/test/browser/browser_experiments.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_experiments.js
@@ -34,16 +34,17 @@ add_task(function* initializeState() {
   // not the superset Experiments Manager has imposed.
   if ("@mozilla.org/browser/experiments-service;1" in Components.classes) {
     let tmp = {};
     Components.utils.import("resource:///modules/experiments/Experiments.jsm", tmp);
     // There is a race condition between XPCOM service initialization and
     // this test running. We have to initialize the instance first, then
     // uninitialize it to prevent this.
     gExperiments = tmp.Experiments.instance();
+    yield gExperiments._mainTask;
     yield gExperiments.uninit();
   }
 });
 
 // On an empty profile with no experiments, the experiment category
 // should be hidden.
 add_task(function* testInitialState() {
   Assert.ok(gCategoryUtilities.get("experiment", false), "Experiment tab is defined.");