diff --git a/mozilla/docshell/base/Makefile.in b/mozilla/docshell/base/Makefile.in
index 248079c6ab9..36bafffcc8f 100644
--- a/mozilla/docshell/base/Makefile.in
+++ b/mozilla/docshell/base/Makefile.in
@@ -83,6 +83,7 @@ REQUIRES	= xpcom \
 		  editor \
 		  windowwatcher \
 		  imglib2 \
+		  mimetype \
 		  $(NULL)
 
 SDK_XPIDLSRCS = \
diff --git a/mozilla/docshell/base/nsWebNavigationInfo.cpp b/mozilla/docshell/base/nsWebNavigationInfo.cpp
index f52ef53841a..a0a475fd33b 100644
--- a/mozilla/docshell/base/nsWebNavigationInfo.cpp
+++ b/mozilla/docshell/base/nsWebNavigationInfo.cpp
@@ -41,6 +41,12 @@
 #include "nsServiceManagerUtils.h"
 #include "nsIDocumentLoaderFactory.h"
 #include "nsIPluginManager.h"
+#include "nsILookAndFeel.h"
+#include "nsWidgetsCID.h"
+#include "nsIMIMEService.h"
+#include "nsIMIMEInfo.h"
+
+static NS_DEFINE_CID(kLookAndFeelCID, NS_LOOKANDFEEL_CID);
 
 NS_IMPL_ISUPPORTS1(nsWebNavigationInfo, nsIWebNavigationInfo)
 
@@ -143,6 +149,37 @@ nsWebNavigationInfo::IsTypeSupportedInternal(const nsCString& aType,
     }
   }
   else if (value.EqualsLiteral(PLUGIN_DLF_CONTRACT)) {
+#ifdef ACCESSIBILITY
+    // If a screen reader is running, prefer external handlers over plugins
+    // because they are better at dealing with accessibility -- they don't
+    // have the keyboard navigation problems and are more likely to expose
+    // their content via MSAA.
+    // XXX Eventually we will remove this once the major content types are as
+    //     screen reader accessible in plugins as in external apps.
+    nsCOMPtr<nsILookAndFeel> lookAndFeel = do_GetService(kLookAndFeelCID);
+    if (lookAndFeel) {
+      PRInt32 isScreenReaderActive;
+      lookAndFeel->GetMetric(nsILookAndFeel::eMetric_IsScreenReaderActive,
+                            isScreenReaderActive);
+      if (isScreenReaderActive) {
+        nsCOMPtr<nsIMIMEService> mimeService(do_GetService("@mozilla.org/mime;1"));
+        if (mimeService) {
+          nsCOMPtr<nsIMIMEInfo> mimeInfo;
+          mimeService->GetFromTypeAndExtension(aType, EmptyCString(),
+                                              getter_AddRefs(mimeInfo));
+          if (mimeInfo) {
+            PRBool hasDefaultHandler;
+            mimeInfo->GetHasDefaultHandler(&hasDefaultHandler);
+            if (hasDefaultHandler) {
+              // External app exists to handle this type, so use it instead of PLUGIN
+              *aIsSupported = nsIWebNavigationInfo::UNSUPPORTED;
+              return NS_OK;
+            }
+          }
+        }
+      }
+    }
+#endif
     *aIsSupported = nsIWebNavigationInfo::PLUGIN;
   }
   else {
diff --git a/mozilla/widget/public/nsILookAndFeel.h b/mozilla/widget/public/nsILookAndFeel.h
index 0f7e8bd4f9f..c6ef3570773 100644
--- a/mozilla/widget/public/nsILookAndFeel.h
+++ b/mozilla/widget/public/nsILookAndFeel.h
@@ -173,6 +173,7 @@ public:
     eMetric_DragThresholdX,                               // begin a drag if the mouse is moved further than the threshold while the button is down
     eMetric_DragThresholdY,
     eMetric_UseAccessibilityTheme,                        // Accessibility theme being used?
+    eMetric_IsScreenReaderActive,                         // Screen reader being used?
 
     eMetric_ScrollArrowStyle,                             // position of scroll arrows in a scrollbar
     eMetric_ScrollSliderStyle,                            // is scroll thumb proportional or fixed?
diff --git a/mozilla/widget/src/qt/nsLookAndFeel.cpp b/mozilla/widget/src/qt/nsLookAndFeel.cpp
index b8ddc556121..f27d4b3a1be 100644
--- a/mozilla/widget/src/qt/nsLookAndFeel.cpp
+++ b/mozilla/widget/src/qt/nsLookAndFeel.cpp
@@ -281,6 +281,7 @@ static const char *metricToString[] = {
     "eMetric_DragThresholdX",
     "eMetric_DragThresholdY",
     "eMetric_UseAccessibilityTheme",
+    "eMetric_IsScreenReaderActive",
 
     "eMetric_ScrollArrowStyle",
     "eMetric_ScrollSliderStyle",
diff --git a/mozilla/widget/src/windows/nsLookAndFeel.cpp b/mozilla/widget/src/windows/nsLookAndFeel.cpp
index 9d7edf14200..3867c935f1e 100644
--- a/mozilla/widget/src/windows/nsLookAndFeel.cpp
+++ b/mozilla/widget/src/windows/nsLookAndFeel.cpp
@@ -392,6 +392,11 @@ NS_IMETHODIMP nsLookAndFeel::GetMetric(const nsMetricID aID, PRInt32 & aMetric)
           aMetric = 0;
         }
         break;
+    case eMetric_IsScreenReaderActive:
+      BOOL isScreenReaderActive;
+      aMetric = SUCCEEDED(SystemParametersInfo(SPI_GETSCREENREADER, 0, &isScreenReaderActive, 0)) && 
+                isScreenReaderActive;
+      break;
 #endif
     case eMetric_ScrollArrowStyle:
         aMetric = eMetric_ScrollArrowStyleSingle;
diff --git a/mozilla/widget/src/xpwidgets/nsXPLookAndFeel.cpp b/mozilla/widget/src/xpwidgets/nsXPLookAndFeel.cpp
index 62d4803d56f..afd159301c1 100644
--- a/mozilla/widget/src/xpwidgets/nsXPLookAndFeel.cpp
+++ b/mozilla/widget/src/xpwidgets/nsXPLookAndFeel.cpp
@@ -91,6 +91,7 @@ nsLookAndFeelIntPref nsXPLookAndFeel::sIntPrefs[] =
   { "ui.dragThresholdX", eMetric_DragThresholdX, PR_FALSE, nsLookAndFeelTypeInt, 0 },
   { "ui.dragThresholdY", eMetric_DragThresholdY, PR_FALSE, nsLookAndFeelTypeInt, 0 },
   { "ui.useAccessibilityTheme", eMetric_UseAccessibilityTheme, PR_FALSE, nsLookAndFeelTypeInt, 0 },
+  { "ui.isScreenReaderActive", eMetric_IsScreenReaderActive, PR_FALSE, nsLookAndFeelTypeInt, 0 },
   { "ui.menusCanOverlapOSBar", eMetric_MenusCanOverlapOSBar,
     PR_FALSE, nsLookAndFeelTypeInt, 0 },
   { "ui.treeOpenDelay",