diff -r 000000000000 -r 6474c204b198 toolkit/components/thumbnails/test/browser_thumbnails_bg_captureIfMissing.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/thumbnails/test/browser_thumbnails_bg_captureIfMissing.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,24 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function runTests() {
+  let url = "http://example.com/";
+  let file = thumbnailFile(url);
+  ok(!file.exists(), "Thumbnail file should not already exist.");
+
+  let capturedURL = yield bgCaptureIfMissing(url);
+  is(capturedURL, url, "Captured URL should be URL passed to capture");
+  ok(file.exists(url), "Thumbnail should be cached after capture");
+
+  let past = Date.now() - 1000000000;
+  let pastFudge = past + 30000;
+  file.lastModifiedTime = past;
+  ok(file.lastModifiedTime < pastFudge, "Last modified time should stick!");
+  capturedURL = yield bgCaptureIfMissing(url);
+  is(capturedURL, url, "Captured URL should be URL passed to second capture");
+  ok(file.exists(), "Thumbnail should remain cached after second capture");
+  ok(file.lastModifiedTime < pastFudge,
+     "File should not have been overwritten");
+
+  file.remove(false);
+}