diff --git a/xpcom/ds/nsCheapSets.h b/xpcom/ds/nsCheapSets.h
index 5e39c88a5d22..4912ae7d81f9 100644
--- a/xpcom/ds/nsCheapSets.h
+++ b/xpcom/ds/nsCheapSets.h
@@ -18,6 +18,7 @@ class nsCheapSet
 {
 public:
   typedef typename EntryType::KeyType KeyType;
+  typedef PLDHashOperator (* Enumerator)(EntryType* aEntry, void* userArg);
 
   nsCheapSet() : mState(ZERO)
   {
@@ -58,6 +59,25 @@ public:
     }
   }
 
+  uint32_t EnumerateEntries(Enumerator enumFunc, void* userArg)
+  {
+    switch (mState) {
+    case ZERO:
+      return 0;
+    case ONE:
+      if (enumFunc(GetSingleEntry(), userArg) == PL_DHASH_REMOVE) {
+        GetSingleEntry()->~EntryType();
+        mState = ZERO;
+      }
+      return 1;
+    case MANY:
+      return mUnion.table->EnumerateEntries(enumFunc, userArg);
+    default:
+      NS_NOTREACHED("bogus state");
+      return 0;
+    }
+  }
+
 private:
   EntryType* GetSingleEntry()
   {