--- a/browser/base/content/test/browser_social.js
+++ b/browser/base/content/test/browser_social.js
@@ -3,16 +3,31 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 // a place for miscellaneous social tests
 
 
 const pb = Cc["@mozilla.org/privatebrowsing;1"].
            getService(Ci.nsIPrivateBrowsingService);
 
+function waitForPortMessage(port, topic, callback) {
+  port.onmessage = function(evt) {
+    if (evt.data.topic == topic)
+      callback(evt.data);
+  }
+}
+
+function portClosed(port) {
+  try {
+    port.postMessage({topic: "ping"});
+    return false; // worked - port can't be closed!
+  } catch (ex) {
+    return true;
+  }
+}
 
 function test() {
   waitForExplicitFinish();
 
   let manifest = { // normal provider
     name: "provider 1",
     origin: "https://example.com",
     sidebarURL: "https://example.com/browser/browser/base/content/test/social_sidebar.html",
@@ -69,22 +84,60 @@ var tests = {
     // disabled through these tests
     Services.obs.addObserver(function observer(aSubject, aTopic) {
       Services.obs.removeObserver(observer, aTopic);
       ok(!Social.enabled, "Social is not enabled");
       togglePrivateBrowsing(function () {
         ok(!Social.enabled, "Social not available during private browsing");
         togglePrivateBrowsing(function () {
           ok(!Social.enabled, "Social is not enabled after private browsing");
-          // social will be reenabled on start of next social test
+          // reenable social for next social test
+          Social.enabled = true;
           next();
         });
       });
     }, "social:pref-changed", false);
-  }
+  },
+
+  testPrivateBrowsingExitReloads: function(next) {
+    let port = Social.provider.getWorkerPort();
+    waitForPortMessage(port, "got-sidebar-message", function(data) {
+      ok(!portClosed(port), "port not closed before PB transition");
+      togglePrivateBrowsing(function () {
+        ok(!Social.enabled, "Social shuts down during private browsing");
+        // check the port we had before is dead.
+        ok(portClosed(port), "port closed after PB transition");
+        // enable it and stick a value in the window object.
+        Social.enabled = true;
+        port = Social.provider.getWorkerPort();
+        waitForPortMessage(port, "got-sidebar-message", function(data) {
+          // now just stuff a value in the sidebar - it should end
+          // up being removed when we leave PB mode via the sidebar
+          // being reloaded.
+          let sbw = document.getElementById("social-sidebar-browser").contentWindow;
+          sbw.wrappedJSObject.foo = "bar";
+          // Now toggle PB mode back to off.
+          togglePrivateBrowsing(function () {
+            ok(Social.enabled, "Social still enabled after leaving private browsing");
+            ok(portClosed(port), "port closed after PB transition");
+            port = Social.provider.getWorkerPort();
+            waitForPortMessage(port, "got-sidebar-message", function() {
+              sbw = document.getElementById("social-sidebar-browser").contentWindow;
+              is(sbw.wrappedJSObject.foo, undefined, "should have lost window variable when exiting")
+              next();
+            });
+            port.postMessage({topic: "test-init"});
+          });
+        });
+        port.postMessage({topic: "test-init"});
+      });
+    });
+    port.postMessage({topic: "test-init"});
+  },
+
 }
 
 function togglePrivateBrowsing(aCallback) {
   Services.obs.addObserver(function observe(subject, topic, data) {
     Services.obs.removeObserver(observe, topic);
     executeSoon(aCallback);
   }, "private-browsing-transition-complete", false);
 

--- a/browser/modules/Social.jsm
+++ b/browser/modules/Social.jsm
@@ -43,16 +43,22 @@ this.Social = {
   },
 
   observe: function(aSubject, aTopic, aData) {
     if (aTopic == "private-browsing") {
       if (aData == "enter") {
         this._enabledBeforePrivateBrowsing = this.enabled;
         this.enabled = false;
       } else if (aData == "exit") {
+        // if the user has explicitly re-enabled social in PB mode, then upon
+        // leaving we want to tear the world down then reenable to prevent
+        // information leaks during this transition.
+        // The next 2 lines rely on the fact that setting this.enabled to
+        // its current value doesn't actually do anything...
+        this.enabled = false;
         this.enabled = this._enabledBeforePrivateBrowsing;
       }
     }
   },
 
   get uiVisible() {
     return this.provider && this.provider.enabled;
   },