diff -r 000000000000 -r 6474c204b198 toolkit/components/thumbnails/test/browser_thumbnails_bg_no_duplicates.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/thumbnails/test/browser_thumbnails_bg_no_duplicates.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,31 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function runTests() {
+  let url = "http://example.com/1";
+  ok(!thumbnailExists(url), "Thumbnail file should not already exist.");
+  let numCallbacks = 0;
+  let doneCallback = function(doneUrl) {
+    is(doneUrl, url, "called back with correct url");
+    numCallbacks += 1;
+    // We will delete the file after the first callback, then check it
+    // still doesn't exist on the second callback, which should give us
+    // confidence that we didn't end up with 2 different captures happening
+    // for the same url...
+    if (numCallbacks == 1) {
+      ok(thumbnailExists(url), "Thumbnail file should now exist.");
+      removeThumbnail(url);
+      return;
+    }
+    if (numCallbacks == 2) {
+      ok(!thumbnailExists(url), "Thumbnail file should still be deleted.");
+      // and that's all we expect, so we are done...
+      next();
+      return;
+    }
+    ok(false, "only expecting 2 callbacks");
+  }
+  BackgroundPageThumbs.capture(url, {onDone: doneCallback});
+  BackgroundPageThumbs.capture(url, {onDone: doneCallback});
+  yield true;
+}