diff --git a/mozilla/xpcom/ds/nsAtomTable.cpp b/mozilla/xpcom/ds/nsAtomTable.cpp
index b2aa909b54f..9ad1536bde0 100644
--- a/mozilla/xpcom/ds/nsAtomTable.cpp
+++ b/mozilla/xpcom/ds/nsAtomTable.cpp
@@ -397,7 +397,7 @@ AtomImpl::~AtomImpl()
   // Permanent atoms are removed from the hashtable at shutdown, and we
   // don't want to remove them twice.  See comment above in
   // |AtomTableClearEntry|.
-  if (!IsPermanent()) {
+  if (!IsPermanentInDestructor()) {
     AtomTableEntry key(mString);
     PL_DHashTableOperate(&gAtomTable, &key, PL_DHASH_REMOVE);
     if (gAtomTable.entryCount == 0) {
@@ -413,6 +413,11 @@ NS_IMPL_THREADSAFE_ISUPPORTS1(AtomImpl, nsIAtom)
 PermanentAtomImpl::PermanentAtomImpl()
   : AtomImpl()
 {
+}
+
+PermanentAtomImpl::~PermanentAtomImpl()
+{
+  // So we can tell if we were permanent while running the base class dtor.
   mRefCnt = REFCNT_PERMANENT_SENTINEL;
 }
 
@@ -426,6 +431,18 @@ NS_IMETHODIMP_(nsrefcnt) PermanentAtomImpl::Release()
   return 1;
 }
 
+/* virtual */ PRBool
+AtomImpl::IsPermanent()
+{
+  return PR_FALSE;
+}
+
+/* virtual */ PRBool
+PermanentAtomImpl::IsPermanent()
+{
+  return PR_TRUE;
+}
+
 void* AtomImpl::operator new ( size_t size, const nsACString& aString ) CPP_THROW_NEW
 {
     /*
diff --git a/mozilla/xpcom/ds/nsAtomTable.h b/mozilla/xpcom/ds/nsAtomTable.h
index 65f170d64cf..53fc5ef3bab 100644
--- a/mozilla/xpcom/ds/nsAtomTable.h
+++ b/mozilla/xpcom/ds/nsAtomTable.h
@@ -61,7 +61,12 @@ public:
 
   enum { REFCNT_PERMANENT_SENTINEL = PR_UINT32_MAX };
 
-  PRBool IsPermanent() { return mRefCnt == REFCNT_PERMANENT_SENTINEL; }
+  virtual PRBool IsPermanent();
+
+  // We can't use the virtual function in the base class destructor.
+  PRBool IsPermanentInDestructor() {
+    return mRefCnt == REFCNT_PERMANENT_SENTINEL;
+  }
 
   void* operator new(size_t size, const nsACString& aString) CPP_THROW_NEW;
 
@@ -84,9 +89,12 @@ public:
 class PermanentAtomImpl : public AtomImpl {
 public:
   PermanentAtomImpl();
+  ~PermanentAtomImpl();
   NS_IMETHOD_(nsrefcnt) AddRef();
   NS_IMETHOD_(nsrefcnt) Release();
 
+  virtual PRBool IsPermanent();
+
   void* operator new(size_t size, const nsACString& aString) CPP_THROW_NEW {
     return AtomImpl::operator new(size, aString);
   }