diff --git a/mozilla/xpcom/base/nsDebug.cpp b/mozilla/xpcom/base/nsDebug.cpp
index 76408c92c02..693f5bf1e65 100644
--- a/mozilla/xpcom/base/nsDebug.cpp
+++ b/mozilla/xpcom/base/nsDebug.cpp
@@ -340,6 +340,7 @@ NS_ErrorAccordingToNSPR()
 #include "prthread.h"
 
 extern "C" NS_EXPORT void* NS_CurrentThread(void);
+extern "C" NS_EXPORT void NS_CheckThreadSafe(void* owningThread, const char* msg);
 
 void*
 NS_CurrentThread(void)
@@ -348,6 +349,29 @@ NS_CurrentThread(void)
   return th;
 }
 
+/*
+ * DON'T TOUCH THAT DIAL...
+ * For now, we're making the thread-safety checking be on by default which, yes, slows
+ * down linux a bit... but we're doing it so that everybody has a chance to exercise 
+ * their code a good deal before the beta. After we branch, we'll turn this back off
+ * and then you can enable it explicitly by setting XPCOM_CHECK_THREADSAFE. This will
+ * let you verify the thread-safety of your classes without impacting everyone all the
+ * time.
+ */
+static PRBool gCheckThreadSafeDefault = PR_TRUE; // READ THE ABOVE COMMENT FIRST!
+
+void
+NS_CheckThreadSafe(void* owningThread, const char* msg)
+{
+  static int check = -1;
+  if (check == -1) {
+    check = gCheckThreadSafeDefault || getenv("XPCOM_CHECK_THREADSAFE") != 0;
+  }
+  if (check) {
+    NS_ASSERTION(owningThread == NS_CurrentThread(), msg);
+  }
+}
+
 #endif // NS_DEBUG
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/mozilla/xpcom/base/nsISupportsUtils.h b/mozilla/xpcom/base/nsISupportsUtils.h
index afc92d0e5f8..d3bda882e84 100644
--- a/mozilla/xpcom/base/nsISupportsUtils.h
+++ b/mozilla/xpcom/base/nsISupportsUtils.h
@@ -142,19 +142,19 @@ public:
 #if defined(NS_DEBUG) && defined(NS_MT_SUPPORTED)
 
 extern "C" NS_EXPORT void* NS_CurrentThread(void);
+extern "C" NS_EXPORT void NS_CheckThreadSafe(void* owningThread, const char* msg);
 
-#define NS_DECL_OWNINGTHREAD     void* _mOwningThread;
-#define NS_IMPL_OWNINGTHREAD()   (_mOwningThread = NS_CurrentThread())
-#define NS_ASSERT_OWNINGTHREAD(_class) \
-    NS_ASSERTION(_mOwningThread == NS_CurrentThread(), #_class " not thread-safe");
+#define NS_DECL_OWNINGTHREAD            void* _mOwningThread;
+#define NS_IMPL_OWNINGTHREAD()          (_mOwningThread = NS_CurrentThread())
+#define NS_ASSERT_OWNINGTHREAD(_class)  NS_CheckThreadSafe(_mOwningThread, #_class " not thread-safe")
 
-#else // !(defined(NS_DEBUG) && !defined(NS_MT_SUPPORTED))
+#else // !(defined(NS_DEBUG) && defined(NS_MT_SUPPORTED))
 
-#define NS_DECL_OWNINGTHREAD     /* nothing */
-#define NS_IMPL_OWNINGTHREAD()   ((void)0)
-#define NS_ASSERT_OWNINGTHREAD(_class) ((void)0)
+#define NS_DECL_OWNINGTHREAD            /* nothing */
+#define NS_IMPL_OWNINGTHREAD()          ((void)0)
+#define NS_ASSERT_OWNINGTHREAD(_class)  ((void)0)
 
-#endif // !(defined(NS_DEBUG) && !defined(NS_MT_SUPPORTED))
+#endif // !(defined(NS_DEBUG) && defined(NS_MT_SUPPORTED))
 
 ////////////////////////////////////////////////////////////////////////////////
 
diff --git a/mozilla/xpcom/build/dlldeps.cpp b/mozilla/xpcom/build/dlldeps.cpp
index 48e05c4d146..fd673fc7655 100644
--- a/mozilla/xpcom/build/dlldeps.cpp
+++ b/mozilla/xpcom/build/dlldeps.cpp
@@ -61,7 +61,6 @@
 #include "nsIInterfaceRequestor.h"
 #ifdef DEBUG
 #include "pure.h"
-extern "C" NS_EXPORT void* NS_CurrentThread(void);
 #endif
 
 class dummyComparitor: public nsAVLNodeComparitor {
@@ -127,7 +126,6 @@ void XXXNeverCalled()
     nsCOMPtr<nsISupports> dummyFoo(do_GetInterface(nsnull));
 #ifdef DEBUG
     TestSegmentedBuffer();
-    NS_CurrentThread();
 #endif
     NS_NewSizeOfHandler(0);
     nsStorageStream();
diff --git a/mozilla/xpcom/glue/nsDebug.cpp b/mozilla/xpcom/glue/nsDebug.cpp
index 76408c92c02..693f5bf1e65 100644
--- a/mozilla/xpcom/glue/nsDebug.cpp
+++ b/mozilla/xpcom/glue/nsDebug.cpp
@@ -340,6 +340,7 @@ NS_ErrorAccordingToNSPR()
 #include "prthread.h"
 
 extern "C" NS_EXPORT void* NS_CurrentThread(void);
+extern "C" NS_EXPORT void NS_CheckThreadSafe(void* owningThread, const char* msg);
 
 void*
 NS_CurrentThread(void)
@@ -348,6 +349,29 @@ NS_CurrentThread(void)
   return th;
 }
 
+/*
+ * DON'T TOUCH THAT DIAL...
+ * For now, we're making the thread-safety checking be on by default which, yes, slows
+ * down linux a bit... but we're doing it so that everybody has a chance to exercise 
+ * their code a good deal before the beta. After we branch, we'll turn this back off
+ * and then you can enable it explicitly by setting XPCOM_CHECK_THREADSAFE. This will
+ * let you verify the thread-safety of your classes without impacting everyone all the
+ * time.
+ */
+static PRBool gCheckThreadSafeDefault = PR_TRUE; // READ THE ABOVE COMMENT FIRST!
+
+void
+NS_CheckThreadSafe(void* owningThread, const char* msg)
+{
+  static int check = -1;
+  if (check == -1) {
+    check = gCheckThreadSafeDefault || getenv("XPCOM_CHECK_THREADSAFE") != 0;
+  }
+  if (check) {
+    NS_ASSERTION(owningThread == NS_CurrentThread(), msg);
+  }
+}
+
 #endif // NS_DEBUG
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/mozilla/xpcom/glue/nsISupportsUtils.h b/mozilla/xpcom/glue/nsISupportsUtils.h
index afc92d0e5f8..d3bda882e84 100644
--- a/mozilla/xpcom/glue/nsISupportsUtils.h
+++ b/mozilla/xpcom/glue/nsISupportsUtils.h
@@ -142,19 +142,19 @@ public:
 #if defined(NS_DEBUG) && defined(NS_MT_SUPPORTED)
 
 extern "C" NS_EXPORT void* NS_CurrentThread(void);
+extern "C" NS_EXPORT void NS_CheckThreadSafe(void* owningThread, const char* msg);
 
-#define NS_DECL_OWNINGTHREAD     void* _mOwningThread;
-#define NS_IMPL_OWNINGTHREAD()   (_mOwningThread = NS_CurrentThread())
-#define NS_ASSERT_OWNINGTHREAD(_class) \
-    NS_ASSERTION(_mOwningThread == NS_CurrentThread(), #_class " not thread-safe");
+#define NS_DECL_OWNINGTHREAD            void* _mOwningThread;
+#define NS_IMPL_OWNINGTHREAD()          (_mOwningThread = NS_CurrentThread())
+#define NS_ASSERT_OWNINGTHREAD(_class)  NS_CheckThreadSafe(_mOwningThread, #_class " not thread-safe")
 
-#else // !(defined(NS_DEBUG) && !defined(NS_MT_SUPPORTED))
+#else // !(defined(NS_DEBUG) && defined(NS_MT_SUPPORTED))
 
-#define NS_DECL_OWNINGTHREAD     /* nothing */
-#define NS_IMPL_OWNINGTHREAD()   ((void)0)
-#define NS_ASSERT_OWNINGTHREAD(_class) ((void)0)
+#define NS_DECL_OWNINGTHREAD            /* nothing */
+#define NS_IMPL_OWNINGTHREAD()          ((void)0)
+#define NS_ASSERT_OWNINGTHREAD(_class)  ((void)0)
 
-#endif // !(defined(NS_DEBUG) && !defined(NS_MT_SUPPORTED))
+#endif // !(defined(NS_DEBUG) && defined(NS_MT_SUPPORTED))
 
 ////////////////////////////////////////////////////////////////////////////////
 
diff --git a/mozilla/xpcom/macbuild/xpcomDebug.exp b/mozilla/xpcom/macbuild/xpcomDebug.exp
index 29c1bb24f14..d370d41bec9 100644
--- a/mozilla/xpcom/macbuild/xpcomDebug.exp
+++ b/mozilla/xpcom/macbuild/xpcomDebug.exp
@@ -1,3 +1,4 @@
 # These are now just additions for the debug build.
 PL_VectorAssertValid
 NS_CurrentThread
+NS_CheckThreadSafe