--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -1607,16 +1607,24 @@ public:
     aWorkerPrivate->OfflineStatusChangeEventInternal(aCx, mIsOffline);
     return true;
   }
 
 private:
   bool mIsOffline;
 };
 
+#ifdef DEBUG
+static bool
+StartsWithExplicit(nsACString& s)
+{
+    return StringBeginsWith(s, NS_LITERAL_CSTRING("explicit/"));
+}
+#endif
+
 class WorkerJSRuntimeStats : public JS::RuntimeStats
 {
   const nsACString& mRtPath;
 
 public:
   explicit WorkerJSRuntimeStats(const nsACString& aRtPath)
   : JS::RuntimeStats(JsWorkerMallocSizeOf), mRtPath(aRtPath)
   { }
@@ -1639,16 +1647,19 @@ public:
   {
     MOZ_ASSERT(!aZoneStats->extra);
 
     // ReportJSRuntimeExplicitTreeStats expects that
     // aZoneStats->extra is a xpc::ZoneStatsExtras pointer.
     xpc::ZoneStatsExtras* extras = new xpc::ZoneStatsExtras;
     extras->pathPrefix = mRtPath;
     extras->pathPrefix += nsPrintfCString("zone(0x%p)/", (void *)aZone);
+
+    MOZ_ASSERT(StartsWithExplicit(extras->pathPrefix));
+
     aZoneStats->extra = extras;
   }
 
   virtual void
   initExtraCompartmentStats(JSCompartment* aCompartment,
                             JS::CompartmentStats* aCompartmentStats)
                             MOZ_OVERRIDE
   {
@@ -1665,16 +1676,19 @@ public:
                                             (void *)js::GetCompartmentZone(aCompartment));
     extras->jsPathPrefix += js::IsAtomsCompartment(aCompartment)
                             ? NS_LITERAL_CSTRING("compartment(web-worker-atoms)/")
                             : NS_LITERAL_CSTRING("compartment(web-worker)/");
 
     // This should never be used when reporting with workers (hence the "?!").
     extras->domPathPrefix.AssignLiteral("explicit/workers/?!/");
 
+    MOZ_ASSERT(StartsWithExplicit(extras->jsPathPrefix));
+    MOZ_ASSERT(StartsWithExplicit(extras->domPathPrefix));
+
     extras->location = nullptr;
 
     aCompartmentStats->extra = extras;
   }
 };
 
 class MessagePortRunnable MOZ_FINAL : public WorkerRunnable
 {
@@ -1900,25 +1914,24 @@ public:
 
   NS_IMETHOD
   CollectReports(nsIMemoryReporterCallback* aCallback,
                  nsISupports* aClosure, bool aAnonymize)
   {
     AssertIsOnMainThread();
 
     // Assumes that WorkerJSRuntimeStats will hold a reference to |path|, and
-    // not a copy, as TryToMapAddon() may later modify if.
+    // not a copy, as TryToMapAddon() may later modify it.
     nsCString path;
     WorkerJSRuntimeStats rtStats(path);
 
     {
       MutexAutoLock lock(mMutex);
 
-      if (!mWorkerPrivate ||
-          !mWorkerPrivate->BlockAndCollectRuntimeStats(&rtStats, aAnonymize)) {
+      if (!mWorkerPrivate) {
         // Returning NS_OK here will effectively report 0 memory.
         return NS_OK;
       }
 
       path.AppendLiteral("explicit/workers/workers(");
       if (aAnonymize && !mWorkerPrivate->Domain().IsEmpty()) {
         path.AppendLiteral("<anonymized-domain>)/worker(<anonymized-url>");
       } else {
@@ -1932,16 +1945,21 @@ public:
         path.AppendLiteral(")/worker(");
         NS_ConvertUTF16toUTF8 escapedURL(mWorkerPrivate->ScriptURL());
         escapedURL.ReplaceChar('/', '\\');
         path.Append(escapedURL);
       }
       path.AppendPrintf(", 0x%p)/", static_cast<void*>(mWorkerPrivate));
 
       TryToMapAddon(path);
+
+      if (!mWorkerPrivate->BlockAndCollectRuntimeStats(&rtStats, aAnonymize)) {
+        // Returning NS_OK here will effectively report 0 memory.
+        return NS_OK;
+      }
     }
 
     return xpc::ReportJSRuntimeExplicitTreeStats(rtStats, path,
                                                  aCallback, aClosure,
                                                  aAnonymize);
   }
 
 private:

--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -2661,18 +2661,17 @@ class OrphanReporter : public JS::Object
   private:
     nsTHashtable <nsISupportsHashKey> mAlreadyMeasuredOrphanTrees;
 };
 
 #ifdef DEBUG
 static bool
 StartsWithExplicit(nsACString& s)
 {
-    const char* e = "explicit/";
-    return Substring(s, 0, strlen(e)).Equals(e);
+    return StringBeginsWith(s, NS_LITERAL_CSTRING("explicit/"));
 }
 #endif
 
 class XPCJSRuntimeStats : public JS::RuntimeStats
 {
     WindowPaths *mWindowPaths;
     WindowPaths *mTopWindowPaths;
     bool mGetLocations;