--- a/netwerk/ipc/NeckoCommon.h
+++ b/netwerk/ipc/NeckoCommon.h
@@ -38,27 +38,48 @@
  *
  * ***** END LICENSE BLOCK ***** */
 
 #ifndef mozilla_net_NeckoCommon_h
 #define mozilla_net_NeckoCommon_h
 
 #include "nsXULAppAPI.h"
 
-// Temporary: use while we figure out which XPCOM interfaces (from list
-// copied from nsHttpChannel) HttpChannel{Child|Parent}, etc., actually need to
-// implement.  I.e., implement interfaces as we need them, and if we never hit a
-// given interface, figure out if we can stop advertising it.
+#if defined(DEBUG) || defined(ENABLE_TESTS)
+# define NECKO_ERRORS_ARE_FATAL_DEFAULT true
+#else
+# define NECKO_ERRORS_ARE_FATAL_DEFAULT false
+#endif 
+
+// TODO: Eventually remove NECKO_MAYBE_ABORT and DROP_DEAD (bug 575494).
+// Still useful for catching listener interfaces we don't yet support across
+// processes, etc.
+
+#define NECKO_MAYBE_ABORT(msg)                                                 \
+  do {                                                                         \
+    bool abort = NECKO_ERRORS_ARE_FATAL_DEFAULT;                               \
+    const char *e = PR_GetEnv("NECKO_ERRORS_ARE_FATAL");                       \
+    if (e)                                                                     \
+      abort = (*e == '0') ? false : true;                                      \
+    if (abort) {                                                               \
+      msg.Append(" (set NECKO_ERRORS_ARE_FATAL=0 in your environment to "      \
+                      "convert this error into a warning.)");                  \
+      NS_RUNTIMEABORT(msg.get());                                              \
+    } else {                                                                   \
+      msg.Append(" (set NECKO_ERRORS_ARE_FATAL=1 in your environment to "      \
+                      "convert this warning into a fatal error.)");            \
+      NS_WARNING(msg.get());                                                   \
+    }                                                                          \
+  } while (0)
 
 #define DROP_DEAD()                                                            \
   do {                                                                         \
-    fprintf(stderr,                                                            \
-            "*&*&*&*&*&*&*&**&*&&*& FATAL ERROR: '%s' UNIMPLEMENTED: %s +%d",  \
-            __FUNCTION__, __FILE__, __LINE__);                                 \
-    NS_ABORT();                                                                \
+    nsPrintfCString msg(1000,"FATAL NECKO ERROR: '%s' UNIMPLEMENTED",          \
+                        __FUNCTION__);                                         \
+    NECKO_MAYBE_ABORT(msg);                                                    \
     return NS_ERROR_NOT_IMPLEMENTED;                                           \
   } while (0)
 
 
 namespace mozilla {
 namespace net {
 
 inline bool 

--- a/netwerk/protocol/http/HttpChannelParent.cpp
+++ b/netwerk/protocol/http/HttpChannelParent.cpp
@@ -276,16 +276,24 @@ HttpChannelParent::OnDataAvailable(nsIRe
 
 //-----------------------------------------------------------------------------
 // HttpChannelParent::nsIInterfaceRequestor
 //-----------------------------------------------------------------------------
 
 NS_IMETHODIMP 
 HttpChannelParent::GetInterface(const nsIID& aIID, void **result)
 {
+  if (aIID.Equals(NS_GET_IID(nsIAuthPromptProvider))) {
+    if (!mTabParent)
+      return NS_NOINTERFACE;
+    return mTabParent->QueryInterface(aIID, result);
+  }
+
+  // TODO: 575494: once we're confident we're handling all needed interfaces,
+  // remove all code below and simply "return QueryInterface(aIID, result)"
   if (// Known interface calls:
 
       // FIXME: HTTP Authorization (bug 537782):
       // nsHttpChannel first tries to get this as an nsIAuthPromptProvider; if that
       // fails, it tries as an nsIAuthPrompt2, and if that fails, an nsIAuthPrompt.
       // See nsHttpChannel::GetAuthPrompt().  So if we can return any one of these,
       // HTTP auth should be all set.  The other two if checks can be eventually
       // deleted.
@@ -302,32 +310,26 @@ HttpChannelParent::GetInterface(const ns
       aIID.Equals(NS_GET_IID(nsIApplicationCacheContainer)) ||
       aIID.Equals(NS_GET_IID(nsIProgressEventSink)) ||
       // FIXME:  bug 561830: when fixed, we shouldn't be asked for this interface
       aIID.Equals(NS_GET_IID(nsIDocShellTreeItem)) ||
       // Let this return NS_ERROR_NO_INTERFACE: it's OK to not provide it.
       aIID.Equals(NS_GET_IID(nsIBadCertListener2))) 
   {
     return QueryInterface(aIID, result);
-  } 
-
-  if (aIID.Equals(NS_GET_IID(nsIAuthPromptProvider))) {
-    if (!mTabParent)
-      return NS_NOINTERFACE;
-    return mTabParent->QueryInterface(aIID, result);
+  } else {
+    nsPrintfCString msg(2000, 
+       "HttpChannelParent::GetInterface: interface UUID=%s not yet supported! "
+       "Use 'grep -ri UUID <mozilla_src>' to find the name of the interface, "
+       "check http://tinyurl.com/255ojvu to see if a bug has already been "
+       "filed, and if not, add one and make it block bug 516730. Thanks!",
+       aIID.ToString());
+    NECKO_MAYBE_ABORT(msg);
+    return NS_NOINTERFACE;
   }
-
-  // Interface we haven't dealt with yet. Make sure we know by dying.
-  // - use "grep -ri [uuid] ROOT_SRC_DIR" with the uuid from the printf to
-  //   find the offending interface.
-  // - FIXME: make non-fatal before we ship
-  printf("*&*&*& HttpChannelParent::GetInterface: uuid=%s not impl'd yet! "
-         "File a bug!\n", 
-         aIID.ToString());
-  DROP_DEAD();
 }
 
 //-----------------------------------------------------------------------------
 // HttpChannelParent::nsIProgressEventSink
 //-----------------------------------------------------------------------------
  
 NS_IMETHODIMP
 HttpChannelParent::OnProgress(nsIRequest *aRequest,