diff --git a/toolkit/components/thumbnails/BackgroundPageThumbs.jsm b/toolkit/components/thumbnails/BackgroundPageThumbs.jsm
index fcfc3973925..ff0d27e0ec7 100644
--- a/toolkit/components/thumbnails/BackgroundPageThumbs.jsm
+++ b/toolkit/components/thumbnails/BackgroundPageThumbs.jsm
@@ -38,17 +38,6 @@ const BackgroundPageThumbs = {
    *                 the queue and started.  Defaults to 30000 (30 seconds).
    */
   capture: function (url, options={}) {
-    if (isPrivateBrowsingActive()) {
-      // There's only one, global private-browsing state shared by all private
-      // windows and the thumbnail browser.  Just as if you log into a site in
-      // one private window you're logged in in all private windows, you're also
-      // logged in in the thumbnail browser.  A crude way to avoid capturing
-      // sites in this situation is to refuse to capture at all when any private
-      // windows are open.  See bug 870179.
-      if (options.onDone)
-        Services.tm.mainThread.dispatch(options.onDone.bind(options, url), 0);
-      return;
-    }
     this._captureQueue = this._captureQueue || [];
     this._capturesByURL = this._capturesByURL || new Map();
     // We want to avoid duplicate captures for the same URL.  If there is an
@@ -239,11 +228,28 @@ Capture.prototype = {
    * @param messageManager  The nsIMessageSender of the thumbnail browser.
    */
   start: function (messageManager) {
-    let timeout = typeof(this.options.timeout) == "number" ? this.options.timeout :
+    // The thumbnail browser uses private browsing mode and therefore shares
+    // browsing state with private windows.  To avoid capturing sites that the
+    // user is logged into in private browsing windows, (1) observe window
+    // openings, and if a private window is opened during capture, discard the
+    // capture when it finishes, and (2) don't start the capture at all if a
+    // private window is open already.
+    Services.ww.registerNotification(this);
+    if (isPrivateBrowsingActive()) {
+      // Captures should always finish asyncly.
+      schedule(() => this._done(null));
+      return;
+    }
+
+    // timeout timer
+    let timeout = typeof(this.options.timeout) == "number" ?
+                  this.options.timeout :
                   DEFAULT_CAPTURE_TIMEOUT;
     this._timeoutTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
-    this._timeoutTimer.initWithCallback(this, timeout, Ci.nsITimer.TYPE_ONE_SHOT);
+    this._timeoutTimer.initWithCallback(this, timeout,
+                                        Ci.nsITimer.TYPE_ONE_SHOT);
 
+    // didCapture registration
     this._msgMan = messageManager;
     this._msgMan.sendAsyncMessage("BackgroundPageThumbs:capture",
                                   { id: this.id, url: this.url });
@@ -268,6 +274,7 @@ Capture.prototype = {
       delete this._msgMan;
     }
     delete this.captureCallback;
+    Services.ww.unregisterNotification(this);
   },
 
   // Called when the didCapture message is received.
@@ -283,6 +290,13 @@ Capture.prototype = {
     this._done(null);
   },
 
+  // Called when the window watcher notifies us.
+  observe: function (subj, topic, data) {
+    if (topic == "domwindowopened" &&
+        PrivateBrowsingUtils.isWindowPrivate(subj))
+      this._privateWinOpenedDuringCapture = true;
+  },
+
   _done: function (data) {
     // Note that _done will be called only once, by either receiveMessage or
     // notify, since it calls destroy, which cancels the timeout timer and
@@ -302,7 +316,7 @@ Capture.prototype = {
       }
     }.bind(this);
 
-    if (!data) {
+    if (!data || this._privateWinOpenedDuringCapture) {
       callOnDones();
       return;
     }
@@ -343,3 +357,7 @@ function isPrivateBrowsingActive() {
       return true;
   return false;
 }
+
+function schedule(callback) {
+  Services.tm.mainThread.dispatch(callback, Ci.nsIThread.DISPATCH_NORMAL);
+}
diff --git a/toolkit/components/thumbnails/test/browser_thumbnails_background.js b/toolkit/components/thumbnails/test/browser_thumbnails_background.js
index 8657033f012..689b0c404b3 100644
--- a/toolkit/components/thumbnails/test/browser_thumbnails_background.js
+++ b/toolkit/components/thumbnails/test/browser_thumbnails_background.js
@@ -164,6 +164,42 @@ let tests = [
     win.close();
   },
 
+  function openPrivateWindowDuringCapture() {
+    let url = "http://example.com/";
+    let file = fileForURL(url);
+    ok(!file.exists(), "Thumbnail file should not already exist.");
+
+    let deferred = imports.Promise.defer();
+
+    let waitCount = 0;
+    function maybeFinish() {
+      if (++waitCount == 2)
+        deferred.resolve();
+    }
+
+    imports.BackgroundPageThumbs.capture(url, {
+      onDone: function (capturedURL) {
+        is(capturedURL, url, "Captured URL should be URL passed to capture.");
+        ok(!file.exists(),
+           "Thumbnail file should not exist because a private window " +
+           "was opened during the capture.");
+        maybeFinish();
+      },
+    });
+
+    // Opening the private window at this point relies on a couple of
+    // implementation details: (1) The capture will start immediately and
+    // synchronously (since at this point in the test, the service is
+    // initialized and its queue is empty), and (2) when it starts the capture
+    // registers with the window watcher.
+    openPrivateWindow().then(function (win) {
+      win.close();
+      maybeFinish();
+    });
+
+    yield deferred.promise;
+  },
+
   function noCookies() {
     // Visit the test page in the browser and tell it to set a cookie.
     let url = testPageURL({ setGreenCookie: true });
@@ -263,7 +299,7 @@ let tests = [
     imports.BackgroundPageThumbs.capture(url, {onDone: doneCallback});
     imports.BackgroundPageThumbs.capture(url, {onDone: doneCallback});
     yield deferred.promise;
-  }
+  },
 ];
 
 function capture(url, options) {