diff -r 000000000000 -r 6474c204b198 startupcache/test/TestStartupCacheTelemetry.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/startupcache/test/TestStartupCacheTelemetry.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,60 @@
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+function shouldHaveChanged(a, b)
+{
+  if (a.length != b.length) {
+    throw Error("TEST-UNEXPECTED-FAIL: telemetry count array size changed");
+  }
+
+  for (let i = 0; i < a.length; ++i) {
+    if (a[i] == b[i]) {
+      continue;
+    }
+    return; // something was different, that's all that matters
+  }
+  throw Error("TEST-UNEXPECTED-FAIL: telemetry data didn't change");
+}
+
+function TestStartupCacheTelemetry() { }
+
+TestStartupCacheTelemetry.prototype = {
+  classID: Components.ID("{73cbeffd-d6c7-42f0-aaf3-f176430dcfc8}"),
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+
+  saveInitial: function() {
+    let t = Services.telemetry;
+    this._age = t.getHistogramById("STARTUP_CACHE_AGE_HOURS").snapshot.counts;
+    this._invalid = t.getHistogramById("STARTUP_CACHE_INVALID").snapshot.counts;
+  },
+
+  checkFinal: function() {
+    let t = Services.telemetry;
+    let newAge = t.getHistogramById("STARTUP_CACHE_AGE_HOURS").snapshot.counts;
+    shouldHaveChanged(this._age, newAge);
+
+    let newInvalid = t.getHistogramById("STARTUP_CACHE_INVALID").snapshot.counts;
+    shouldHaveChanged(this._invalid, newInvalid);
+  },
+
+  observe: function(subject, topic, data) {
+    switch (topic) {
+    case "save-initial":
+      this.saveInitial();
+      break;
+
+    case "check-final":
+      this.checkFinal();
+      break;
+
+    default:
+      throw Error("BADDOG, NO MILKBONE FOR YOU");
+    }
+  },
+};
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([TestStartupCacheTelemetry]);