diff --git a/browser/components/privatebrowsing/src/nsPrivateBrowsingService.js b/browser/components/privatebrowsing/src/nsPrivateBrowsingService.js
index 4a3086727e25..e7cd108d0b4b 100644
--- a/browser/components/privatebrowsing/src/nsPrivateBrowsingService.js
+++ b/browser/components/privatebrowsing/src/nsPrivateBrowsingService.js
@@ -101,12 +101,19 @@ PrivateBrowsingService.prototype = {
            .usePrivateBrowsing = aFlag;
   },
 
-  _onBeforePrivateBrowsingModeChange: function PBS__onBeforePrivateBrowsingModeChange() {
+  _adjustPBFlagOnExistingWindows: function PBS__adjustPBFlagOnExistingWindows() {
     var windowsEnum = Services.wm.getEnumerator(null);
     while (windowsEnum.hasMoreElements()) {
       var window = windowsEnum.getNext();
       this._setPerWindowPBFlag(window, this._inPrivateBrowsing);
     }
+  },
+
+  _onBeforePrivateBrowsingModeChange: function PBS__onBeforePrivateBrowsingModeChange() {
+    // If we're about to enter PB mode, adjust the flags now
+    if (this._inPrivateBrowsing) {
+      this._adjustPBFlagOnExistingWindows();
+    }
 
     // nothing needs to be done here if we're enabling at startup
     if (!this._autoStarted) {
@@ -180,6 +187,11 @@ PrivateBrowsingService.prototype = {
     }
     else
       this._saveSession = false;
+
+    // If we're about to leave PB mode, adjust the flags now
+    if (!this._inPrivateBrowsing) {
+      this._adjustPBFlagOnExistingWindows();
+    }
   },
 
   _onAfterPrivateBrowsingModeChange: function PBS__onAfterPrivateBrowsingModeChange() {
diff --git a/toolkit/components/places/AsyncFaviconHelpers.cpp b/toolkit/components/places/AsyncFaviconHelpers.cpp
index 3e82dd272dc2..01ca6933fba0 100644
--- a/toolkit/components/places/AsyncFaviconHelpers.cpp
+++ b/toolkit/components/places/AsyncFaviconHelpers.cpp
@@ -515,7 +515,18 @@ AsyncFetchAndSetIconForPage::AsyncFetchAndSetIconForPage(
   if (pbService) {
     bool inPrivateBrowsing = false;
     if (NS_SUCCEEDED(pbService->GetPrivateBrowsingEnabled(&inPrivateBrowsing))) {
-      MOZ_ASSERT(inPrivateBrowsing == mFaviconLoadPrivate);
+      // In one specific case that we know of, it is possible for these flags
+      // to not match (bug 801151).  We mostly care about the cases where the
+      // global private browsing mode is on, but the favicon load is not marked
+      // as private, as those cases will cause privacy leaks.  But because
+      // fixing bug 801151 properly is going to mean tons of really dirty and
+      // fragile work which might cause other types of problems, we fatally
+      // assert the condition which would be a privacy leak, and non-fatally
+      // assert the other side of the condition which would designate a bug,
+      // but not a privacy sensitive one.
+      MOZ_ASSERT_IF(inPrivateBrowsing && !mFaviconLoadPrivate, false);
+      NS_ASSERTION(inPrivateBrowsing == mFaviconLoadPrivate,
+                   "The favicon load flag and the global PB flag do not match");
     }
   }
 #endif