diff --git a/modules/libpref/src/init/all.js b/modules/libpref/src/init/all.js
index f622a81414a..1dd8e3df997 100644
--- a/modules/libpref/src/init/all.js
+++ b/modules/libpref/src/init/all.js
@@ -713,7 +713,6 @@ pref("network.ntlm.send-lm-response", false);
 pref("network.hosts.nntp_server",           "news.mozilla.org");
 
 pref("permissions.default.image",           1); // 1-Accept, 2-Deny, 3-dontAcceptForeign
-pref("network.proxy.autoconfig_url",        "");
 pref("network.proxy.type",                  0);
 pref("network.proxy.ftp",                   "");
 pref("network.proxy.ftp_port",              0);
@@ -737,6 +736,14 @@ pref("network.cookie.alwaysAcceptSessionCookies", false);
 pref("network.cookie.prefsMigrated",        false);
 pref("network.cookie.lifetime.days",        90);
 
+// The PAC file to load.  Ignored unless network.proxy.type is 2.
+pref("network.proxy.autoconfig_url", "");
+
+// If we cannot load the PAC file, then try again (doubling from interval_min
+// until we reach interval_max or the PAC file is successfully loaded).
+pref("network.proxy.autoconfig_retry_interval_min", 5);    // 5 seconds
+pref("network.proxy.autoconfig_retry_interval_max", 300);  // 5 minutes
+
 // The following default value is for p3p medium mode.
 // See xpfe/components/permissions/content/cookieP3P.xul for the definitions of low/medium/hi
 pref("network.cookie.p3p",                  "ffffaaaa");
diff --git a/netwerk/base/src/nsPACMan.cpp b/netwerk/base/src/nsPACMan.cpp
index c6f6aeb78e9..de8d01a0bc7 100644
--- a/netwerk/base/src/nsPACMan.cpp
+++ b/netwerk/base/src/nsPACMan.cpp
@@ -41,15 +41,37 @@
 #include "nsIDNSService.h"
 #include "nsIDNSListener.h"
 #include "nsICancelable.h"
+#include "nsIAuthPrompt.h"
+#include "nsIHttpChannel.h"
+#include "nsIPrefService.h"
+#include "nsIPrefBranch.h"
 #include "nsNetUtil.h"
 #include "nsAutoLock.h"
 #include "nsAutoPtr.h"
-#include "nsIAuthPrompt.h"
 #include "nsCRT.h"
 #include "prmon.h"
 
 //-----------------------------------------------------------------------------
 
+// Check to see if the underlying request was not an error page in the case of
+// a HTTP request.  For other types of channels, just return true.
+static PRBool
+HttpRequestSucceeded(nsIStreamLoader *loader)
+{
+  nsCOMPtr<nsIRequest> request;
+  loader->GetRequest(getter_AddRefs(request));
+
+  PRBool result = PR_TRUE;  // default to assuming success
+
+  nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(request);
+  if (httpChannel)
+    httpChannel->GetRequestSucceeded(&result);
+
+  return result;
+}
+
+//-----------------------------------------------------------------------------
+
 // These objects are stored in nsPACMan::mPendingQ
 
 class PendingPACQuery : public PRCList, public nsIDNSListener
@@ -150,6 +172,8 @@ PendingPACQuery::OnLookupComplete(nsICancelable *request,
 nsPACMan::nsPACMan()
   : mLoadPending(PR_FALSE)
   , mShutdown(PR_FALSE)
+  , mScheduledReload(LL_MAXINT)
+  , mLoadFailureCount(0)
 {
   PR_INIT_CLIST(&mPendingQ);
 }
@@ -181,6 +205,8 @@ nsPACMan::GetProxyForURI(nsIURI *uri, nsACString &result)
     return NS_OK;
   }
 
+  MaybeReloadPAC();
+
   if (IsLoading())
     return NS_ERROR_IN_PROGRESS;
   if (!mPAC)
@@ -198,6 +224,8 @@ nsPACMan::AsyncGetProxyForURI(nsIURI *uri, nsPACManCallback *callback)
 {
   NS_ENSURE_STATE(!mShutdown);
 
+  MaybeReloadPAC();
+
   PendingPACQuery *query = new PendingPACQuery(this, uri, callback);
   if (!query)
     return NS_ERROR_OUT_OF_MEMORY;
@@ -226,6 +254,7 @@ nsresult
 nsPACMan::LoadPACFromURI(nsIURI *pacURI)
 {
   NS_ENSURE_STATE(!mShutdown);
+  NS_ENSURE_ARG(pacURI || mPACURI);
 
   nsCOMPtr<nsIStreamLoader> loader =
       do_CreateInstance(NS_STREAMLOADER_CONTRACTID);
@@ -249,7 +278,11 @@ nsPACMan::LoadPACFromURI(nsIURI *pacURI)
   CancelExistingLoad();
 
   mLoader = loader;
-  mPACURI = pacURI;
+  if (pacURI) {
+    mPACURI = pacURI;
+    mLoadFailureCount = 0;  // reset
+  }
+  mScheduledReload = LL_MAXINT;
   mPAC = nsnull;
   return NS_OK;
 }
@@ -285,6 +318,43 @@ nsPACMan::StartLoading()
   ProcessPendingQ(NS_ERROR_UNEXPECTED);
 }
 
+void
+nsPACMan::MaybeReloadPAC()
+{
+  if (!mPACURI)
+    return;
+
+  if (PR_Now() > mScheduledReload) {
+    printf("\n>>> reloading PAC <<<\n\n");
+    LoadPACFromURI(nsnull);
+  }
+}
+
+void
+nsPACMan::OnLoadFailure()
+{
+  PRInt32 minInterval = 5;    // 5 seconds
+  PRInt32 maxInterval = 300;  // 5 minutes
+
+  nsCOMPtr<nsIPrefBranch> prefs = do_GetService(NS_PREFSERVICE_CONTRACTID);
+  if (prefs) {
+    prefs->GetIntPref("network.proxy.autoconfig_retry_interval_min",
+                      &minInterval);
+    prefs->GetIntPref("network.proxy.autoconfig_retry_interval_max",
+                      &maxInterval);
+  }
+
+  PRInt32 interval = minInterval << mLoadFailureCount++;  // seconds
+  if (!interval || interval > maxInterval)
+    interval = maxInterval;
+
+#ifdef DEBUG
+  printf("PAC load failure: will retry in %d seconds\n", interval);
+#endif
+
+  mScheduledReload = PR_Now() + PRInt64(interval) * PR_USEC_PER_SEC;
+}
+
 void
 nsPACMan::CancelExistingLoad()
 {
@@ -340,7 +410,7 @@ nsPACMan::OnStreamComplete(nsIStreamLoader *loader,
 
   mLoader = nsnull;
 
-  if (NS_SUCCEEDED(status)) {
+  if (NS_SUCCEEDED(status) && HttpRequestSucceeded(loader)) {
     // Get the URI spec used to load this PAC script.
     nsCAutoString pacURI;
     {
@@ -367,6 +437,14 @@ nsPACMan::OnStreamComplete(nsIStreamLoader *loader,
       const char *text = (const char *) data;
       status = mPAC->Init(pacURI, NS_ConvertASCIItoUTF16(text, dataLen));
     }
+
+    // Even if the PAC file could not be parsed, we did succeed in loading the
+    // data for it.
+    mLoadFailureCount = 0;
+  } else {
+    // We were unable to load the PAC file (presumably because of a network
+    // failure).  Try again a little later.
+    OnLoadFailure();
   }
 
   // Reset mPAC if necessary
diff --git a/netwerk/base/src/nsPACMan.h b/netwerk/base/src/nsPACMan.h
index 0df01211a0c..f977cd80d77 100644
--- a/netwerk/base/src/nsPACMan.h
+++ b/netwerk/base/src/nsPACMan.h
@@ -118,7 +118,8 @@ public:
    * processed once the PAC file finishes loading.
    *
    * @param pacURI
-   *        The nsIURI of the PAC file to load.
+   *        The nsIURI of the PAC file to load.  If this parameter is null,
+   *        then the previous PAC URI is simply reloaded.
    */
   nsresult LoadPACFromURI(nsIURI *pacURI);
 
@@ -151,6 +152,16 @@ private:
    */
   void StartLoading();
 
+  /**
+   * Reload the PAC file if there is reason to.
+   */
+  void MaybeReloadPAC();
+
+  /**
+   * Called when we fail to load the PAC file.
+   */
+  void OnLoadFailure();
+
   /**
    * Returns true if the given URI matches the URI of our PAC file.
    */
@@ -166,6 +177,8 @@ private:
   nsCOMPtr<nsIStreamLoader>    mLoader;
   PRPackedBool                 mLoadPending;
   PRPackedBool                 mShutdown;
+  PRTime                       mScheduledReload;
+  PRUint32                     mLoadFailureCount;
 };
 
 #endif  // nsPACMan_h__