--- a/image/src/DiscardTracker.cpp
+++ b/image/src/DiscardTracker.cpp
@@ -148,22 +148,22 @@ DiscardTracker::Initialize()
 
   Preferences::AddUintVarCache(&sMaxDecodedImageKB,
                               "image.mem.max_decoded_image_kb",
                               50 * 1024);
 
   // Create the timer.
   sTimer = do_CreateInstance("@mozilla.org/timer;1");
 
+  // Mark us as initialized
+  sInitialized = true;
+
   // Read the timeout pref and start the timer.
   ReloadTimeout();
 
-  // Mark us as initialized
-  sInitialized = true;
-
   return NS_OK;
 }
 
 /**
  * Read the discard timeout from about:config.
  */
 void
 DiscardTracker::ReloadTimeout()
@@ -189,19 +189,22 @@ DiscardTracker::ReloadTimeout()
 }
 
 /**
  * Enables the timer. No-op if the timer is already running.
  */
 nsresult
 DiscardTracker::EnableTimer()
 {
-  // Nothing to do if the timer's already on.
-  if (sTimerOn)
+  // Nothing to do if the timer's already on or we haven't yet been
+  // initialized.  !sTimer probably means we've shut down, so just ignore that,
+  // too.
+  if (sTimerOn || !sInitialized || !sTimer)
     return NS_OK;
+
   sTimerOn = true;
 
   // Activate the timer.  Have it call us back in (sMinDiscardTimeoutMs / 2)
   // ms, so that an image is discarded between sMinDiscardTimeoutMs and
   // (3/2 * sMinDiscardTimeoutMs) ms after it's unlocked.
   return sTimer->InitWithFuncCallback(TimerCallback,
                                       nsnull,
                                       sMinDiscardTimeoutMs / 2,
@@ -210,17 +213,17 @@ DiscardTracker::EnableTimer()
 
 /*
  * Disables the timer. No-op if the timer isn't running.
  */
 void
 DiscardTracker::DisableTimer()
 {
   // Nothing to do if the timer's already off.
-  if (!sTimerOn)
+  if (!sTimerOn || !sTimer)
     return;
   sTimerOn = false;
 
   // Deactivate
   sTimer->Cancel();
 }
 
 /**