--- a/startupcache/test/Makefile.in
+++ b/startupcache/test/Makefile.in
@@ -1,6 +1,10 @@
 #
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 LIBS += $(MOZ_COMPONENT_LIBS)
+DIST_FILES += \
+  $(srcdir)/TestStartupCacheTelemetry.manifest \
+  $(srcdir)/TestStartupCacheTelemetry.js \
+  $(NULL)

--- a/startupcache/test/TestStartupCache.cpp
+++ b/startupcache/test/TestStartupCache.cpp
@@ -15,19 +15,17 @@
 #include "nsIStringStream.h"
 #include "nsIStorageStream.h"
 #include "nsIObjectInputStream.h"
 #include "nsIObjectOutputStream.h"
 #include "nsIURI.h"
 #include "nsStringAPI.h"
 #include "nsIPrefBranch.h"
 #include "nsIPrefService.h"
-#include "nsITelemetry.h"
 #include "nsIXPConnect.h"
-#include "jsapi.h"
 #include "prio.h"
 #include "mozilla/Maybe.h"
 
 using namespace JS;
 
 namespace mozilla {
 namespace scache {
 
@@ -395,164 +393,52 @@ TestEarlyShutdown() {
   } else {
     fail("PutBuffer gave an unexpected failure, expected NOT_AVAILABLE");
     return rv;
   }
  
   return NS_OK;
 }
 
-static bool
-GetHistogramCounts(const char *testmsg, const nsACString &histogram_id,
-                   JSContext *cx, MutableHandle<Value> counts)
-{
-  nsCOMPtr<nsITelemetry> telemetry = do_GetService("@mozilla.org/base/telemetry;1");
-  Rooted<Value> h(cx);
-  nsresult trv = telemetry->GetHistogramById(histogram_id, cx, h.address());
-  if (NS_FAILED(trv)) {
-    fail("%s: couldn't get histogram %s", testmsg, ToNewCString(histogram_id));
-    return false;
-  }
-  passed(testmsg);
-
-  Rooted<Value> snapshot_val(cx);
-  JSFunction *snapshot_fn = nullptr;
-  Rooted<Value> ss(cx);
-  return (JS_GetProperty(cx, JSVAL_TO_OBJECT(h), "snapshot",
-                         &snapshot_val)
-          && (snapshot_fn = JS_ValueToFunction(cx, snapshot_val))
-          && JS::Call(cx, JSVAL_TO_OBJECT(h),
-                      snapshot_fn, 0, nullptr, &ss)
-          && JS_GetProperty(cx, JSVAL_TO_OBJECT(ss), "counts", counts));
-}
-
-nsresult
-CompareCountArrays(JSContext *cx, JSObject *aBefore, JSObject *aAfter)
-{
-  uint32_t before_size, after_size;
-  JS::RootedObject before(cx, aBefore);
-  JS::RootedObject after(cx, aAfter);
-  if (!(JS_GetArrayLength(cx, before, &before_size)
-        && JS_GetArrayLength(cx, after, &after_size))) {
-    return NS_ERROR_UNEXPECTED;
-  }
-
-  if (before_size != after_size) {
-    return NS_ERROR_UNEXPECTED;
-  }
-
-  JS::RootedValue before_num(cx), after_num(cx);
-  for (uint32_t i = 0; i < before_size; ++i) {
-    if (!(JS_GetElement(cx, before, i, &before_num)
-          && JS_GetElement(cx, after, i, &after_num))) {
-      return NS_ERROR_UNEXPECTED;
-    }
-
-    bool same = true;
-    if (!JS_LooselyEqual(cx, before_num, after_num, &same)) {
-      return NS_ERROR_UNEXPECTED;
-    } else {
-      if (same) {
-        continue;
-      } else {
-        // Some element of the histograms's count arrays differed.
-        // That's a good thing!
-        return NS_OK;
-      }
-    }
-  }
-
-  // None of the elements of the histograms's count arrays differed.
-  // Not good, we should have recorded something.
-  return NS_ERROR_FAILURE;
-}
-
-nsresult
-TestHistogramValues(const char* type, bool use_js, JSContext *cx,
-                    JSObject *before, JSObject *after)
-{
-  if (!use_js) {
-    fail("couldn't check histogram recording");
-    return NS_ERROR_FAILURE;
-  }
-  nsresult compare = CompareCountArrays(cx, before, after);
-  if (compare == NS_ERROR_UNEXPECTED) {
-    fail("count comparison error");
-    return compare;
-  }
-  if (compare == NS_ERROR_FAILURE) {
-    fail("histogram didn't record %s", type);
-    return compare;
-  }
-  passed("histogram records %s", type);
-  return NS_OK;
-}
-
 int main(int argc, char** argv)
 {
   ScopedXPCOM xpcom("Startup Cache");
   if (xpcom.failed())
     return 1;
 
   nsCOMPtr<nsIPrefBranch> prefs = do_GetService(NS_PREFSERVICE_CONTRACTID);
   prefs->SetIntPref("hangmonitor.timeout", 0);
   
   int rv = 0;
-  // nsITelemetry doesn't have a nice C++ interface.
-  JSContext *cx = nullptr;
+  nsresult scrv;
 
-  // XPCOM initialization spins up XPConnect, which spins up a JSRuntime, which
-  // we can only have one of per thread. So we need to get a JSContext out of
-  // XPConnect here, rather than creating our own runtime. XPConnect rules
-  // dictate that we push the context as well, but we're trying to make the
-  // pushing/popping APIs accessible only through nsCxPusher, which isn't
-  // accessible via the external linkage used by this test. We can get away with
-  // using the cx here without triggering a cx stack assert, so just do that
-  // for now. Eventually, the whole notion of pushing and popping will just go
-  // away.
-  nsCOMPtr<nsIXPConnect> xpc = do_GetService(nsIXPConnect::GetCID());
-  if (xpc)
-    cx = xpc->GetSafeJSContext();
+  // Register TestStartupCacheTelemetry
+  nsCOMPtr<nsIFile> manifest;
+  scrv = NS_GetSpecialDirectory(NS_GRE_DIR,
+                                getter_AddRefs(manifest));
+  if (NS_FAILED(scrv)) {
+    fail("NS_XPCOM_CURRENT_PROCESS_DIR");
+    return 1;
+  }
 
-  bool use_js = !!cx;
-  JSAutoRequest req(cx);
-  static const JSClass global_class = {
-    "global", JSCLASS_NEW_RESOLVE | JSCLASS_GLOBAL_FLAGS | JSCLASS_HAS_PRIVATE,
-    JS_PropertyStub,  JS_DeletePropertyStub,
-    JS_PropertyStub,  JS_StrictPropertyStub,
-    JS_EnumerateStub, JS_ResolveStub,
-    JS_ConvertStub
-  };
-  JSObject *glob = nullptr;
-  if (use_js)
-    glob = JS_NewGlobalObject(cx, &global_class, nullptr, JS::FireOnNewGlobalHook);
-  if (!glob)
-    use_js = false;
-  mozilla::Maybe<JSAutoCompartment> ac;
-  if (use_js)
-    ac.construct(cx, glob);
-  if (use_js && !JS_InitStandardClasses(cx, glob))
-    use_js = false;
+  manifest->AppendNative(NS_LITERAL_CSTRING("TestStartupCacheTelemetry.manifest"));
+  XRE_AddManifestLocation(NS_COMPONENT_LOCATION, manifest);
 
-  NS_NAMED_LITERAL_CSTRING(age_histogram_id, "STARTUP_CACHE_AGE_HOURS");
-  NS_NAMED_LITERAL_CSTRING(invalid_histogram_id, "STARTUP_CACHE_INVALID");
+  nsCOMPtr<nsIObserver> telemetryThing =
+    do_GetService("@mozilla.org/testing/startup-cache-telemetry.js");
+  if (!telemetryThing) {
+    fail("telemetryThing");
+    return 1;
+  }
+  scrv = telemetryThing->Observe(nullptr, "save-initial", nullptr);
+  if (NS_FAILED(scrv)) {
+    fail("save-initial");
+    rv = 1;
+  }
 
-  Rooted<Value> age_before_counts(cx);
-  if (use_js &&
-      !GetHistogramCounts("STARTUP_CACHE_AGE_HOURS histogram before test",
-                          age_histogram_id, cx, &age_before_counts))
-    use_js = false;
-  
-  Rooted<Value> invalid_before_counts(cx);
-  if (use_js &&
-      !GetHistogramCounts("STARTUP_CACHE_INVALID histogram before test",
-                          invalid_histogram_id, cx, &invalid_before_counts))
-    use_js = false;
-  
-  nsresult scrv;
   nsCOMPtr<nsIStartupCache> sc 
     = do_GetService("@mozilla.org/startupcache/cache;1", &scrv);
   if (NS_FAILED(scrv))
     rv = 1;
   else
     sc->RecordAgesAlways();
   if (NS_FAILED(TestStartupWriteRead()))
     rv = 1;
@@ -561,33 +447,16 @@ int main(int argc, char** argv)
   if (NS_FAILED(TestWriteObject()))
     rv = 1;
   nsCOMPtr<nsIFile> profileDir = xpcom.GetProfileDirectory();
   if (NS_FAILED(TestIgnoreDiskCache(profileDir)))
     rv = 1;
   if (NS_FAILED(TestEarlyShutdown()))
     rv = 1;
 
-  Rooted<Value> age_after_counts(cx);
-  if (use_js &&
-      !GetHistogramCounts("STARTUP_CACHE_AGE_HOURS histogram after test",
-                          age_histogram_id, cx, &age_after_counts))
-    use_js = false;
-
-  if (NS_FAILED(TestHistogramValues("age samples", use_js, cx,
-                                    age_before_counts.toObjectOrNull(),
-                                    age_after_counts.toObjectOrNull())))
+  scrv = telemetryThing->Observe(nullptr, "save-initial", nullptr);
+  if (NS_FAILED(scrv)) {
+    fail("check-final");
     rv = 1;
-                                                    
-  Rooted<Value> invalid_after_counts(cx);
-  if (use_js &&
-      !GetHistogramCounts("STARTUP_CACHE_INVALID histogram after test",
-                          invalid_histogram_id, cx, &invalid_after_counts))
-    use_js = false;
-
-  // STARTUP_CACHE_INVALID should have been triggered by TestIgnoreDiskCache()
-  if (NS_FAILED(TestHistogramValues("invalid disk cache", use_js, cx,
-                                    invalid_before_counts.toObjectOrNull(),
-                                    invalid_after_counts.toObjectOrNull())))
-    rv = 1;
+  }
 
   return rv;
 }

new file mode 100644
--- /dev/null
+++ b/startupcache/test/TestStartupCacheTelemetry.js
@@ -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]);

new file mode 100644
--- /dev/null
+++ b/startupcache/test/TestStartupCacheTelemetry.manifest
@@ -0,0 +1,2 @@
+component {73cbeffd-d6c7-42f0-aaf3-f176430dcfc8} TestStartupCacheTelemetry.js
+contract @mozilla.org/testing/startup-cache-telemetry.js {73cbeffd-d6c7-42f0-aaf3-f176430dcfc8}