diff --git a/xpfe/components/prefwindow/resources/content/nsPrefWindow.js b/xpfe/components/prefwindow/resources/content/nsPrefWindow.js
index ff243dbc8fbc..fa342d202865 100644
--- a/xpfe/components/prefwindow/resources/content/nsPrefWindow.js
+++ b/xpfe/components/prefwindow/resources/content/nsPrefWindow.js
@@ -86,17 +86,16 @@ nsPrefWindow.prototype =
       onOK:
         function ()
           {
-            for( var i = 0; i < hPrefWindow.okHandlers.length; i++ )
-              {
-                hPrefWindow.okHandlers[i]();
-              }
-              
             var tag = document.getElementById( hPrefWindow.contentFrame ).getAttribute("tag");
             if( tag == "" )
               {
                 tag = document.getElementById( hPrefWindow.contentFrame ).getAttribute("src");
               }
             hPrefWindow.wsm.savePageData( tag );
+            for( var i = 0; i < hPrefWindow.okHandlers.length; i++ )
+              {
+                hPrefWindow.okHandlers[i]();
+              }
             hPrefWindow.savePrefs();
           },
         
diff --git a/xpfe/components/prefwindow/resources/content/pref-navigator.xul b/xpfe/components/prefwindow/resources/content/pref-navigator.xul
index 2522095d48d8..59257f7e3118 100644
--- a/xpfe/components/prefwindow/resources/content/pref-navigator.xul
+++ b/xpfe/components/prefwindow/resources/content/pref-navigator.xul
@@ -23,6 +23,7 @@
 
 <?xml-stylesheet href="chrome://communicator/skin/" type="text/css"?>
 <?xul-overlay href="chrome://communicator/content/communicatorOverlay.xul"?>
+<?xul-overlay href="chrome://communicator/content/pref/platformPrefOverlay.xul"?>
 
 <!DOCTYPE window SYSTEM "chrome://communicator/locale/pref/pref-navigator.dtd" >
 
@@ -43,16 +44,18 @@
   ]]>
   </script>
   
-  <!-- navigator starts with -->  
-  <groupbox>
-    <caption label="&navRadio;"/>
-    <radiogroup id="startupPage" prefstring="browser.startup.page">
-      <radio value="0" label="&blankPageRadio.label;" accesskey="&blankPageRadio.accesskey;"/>
-      <radio value="1" label="&homePageRadio.label;" accesskey="&homePageRadio.accesskey;"/>
-      <radio value="2" label="&lastPageRadio.label;" accesskey="&lastPageRadio.accesskey;"/>
-    </radiogroup>
-
-  </groupbox>
+  <hbox id="pref-nav-platform-extensions">
+    <!-- navigator starts with -->  
+    <groupbox>
+      <caption label="&navRadio;"/>
+      <radiogroup id="startupPage" prefstring="browser.startup.page">
+        <radio value="0" label="&blankPageRadio.label;" accesskey="&blankPageRadio.accesskey;"/>
+        <radio value="1" label="&homePageRadio.label;" accesskey="&homePageRadio.accesskey;"/>
+        <radio value="2" label="&lastPageRadio.label;" accesskey="&lastPageRadio.accesskey;"/>
+      </radiogroup>
+  
+    </groupbox>
+  </hbox>
 
   <!-- homepage specification -->
   <groupbox>
diff --git a/xpfe/components/prefwindow/resources/content/pref-winhooks.js b/xpfe/components/prefwindow/resources/content/pref-winhooks.js
index edad680b1775..c2e5df04b393 100644
--- a/xpfe/components/prefwindow/resources/content/pref-winhooks.js
+++ b/xpfe/components/prefwindow/resources/content/pref-winhooks.js
@@ -35,23 +35,8 @@ function dumpObject( obj, name ) {
 // Top-level windows integration preferences.
 if ( !( "winHooks" in parent ) ) {
     parent.winHooks = new Object;
-    parent.winHooks.settings = [ "isHandlingHTML",
-                                 "isHandlingJPEG",
-                                 "isHandlingGIF",
-                                 "isHandlingPNG",
-                                 "isHandlingMNG",
-                                 "isHandlingBMP",
-                                 "isHandlingICO",
-                                 "isHandlingXML",
-                                 "isHandlingXHTML",
-                                 "isHandlingXUL",
-                                 "isHandlingHTTP",
-                                 "isHandlingHTTPS",
-                                 "isHandlingFTP",
-                                 "isHandlingCHROME",
-                                 "isHandlingGOPHER",
-                                 "showDialog" ];
     
+    parent.winHooks.settings = null;
     parent.winHooks.winhooks = null;
     parent.winHooks.prefs = null;
 }
@@ -104,14 +89,34 @@ function Startup() {
                 // Set globals.
                 parent.winHooks.winhooks = winhooks;
                 parent.winHooks.prefs    = prefs;
-                // Register so we get called when pref window Ok is pressed.
-                parent.hPrefWindow.registerOKCallbackFunc( onOK );
             }
         }
         catch(e) {
             dump( e + "\n" );
         }
     }         
+    if ( !settings ) {
+        // Set state specific to this panel (not shared with the "default browser"
+        // button state from the Navigator panel).
+        settings = parent.winHooks.settings = [ "isHandlingHTML",
+                                                "isHandlingJPEG",
+                                                "isHandlingGIF",
+                                                "isHandlingPNG",
+                                                "isHandlingMNG",
+                                                "isHandlingBMP",
+                                                "isHandlingICO",
+                                                "isHandlingXML",
+                                                "isHandlingXHTML",
+                                                "isHandlingXUL",
+                                                "isHandlingHTTP",
+                                                "isHandlingHTTPS",
+                                                "isHandlingFTP",
+                                                "isHandlingCHROME",
+                                                "isHandlingGOPHER",
+                                                "showDialog" ];
+        // Register so we get called when pref window Ok is pressed.
+        parent.hPrefWindow.registerOKCallbackFunc( onOK );
+    }
     // Transfer object settings to the dialog checkboxes.
     for( var index in settings  ) {
         var setting = settings[ index ];
diff --git a/xpfe/components/prefwindow/resources/content/win/platformPrefOverlay.xul b/xpfe/components/prefwindow/resources/content/win/platformPrefOverlay.xul
index 187ccecf8fbd..c47c233c94f1 100644
--- a/xpfe/components/prefwindow/resources/content/win/platformPrefOverlay.xul
+++ b/xpfe/components/prefwindow/resources/content/win/platformPrefOverlay.xul
@@ -21,9 +21,15 @@
    
    Contributor(s): 
      Ben Goodger <ben@netscape.com>
+     Bill Law    <law@netscape.com>
   -->
 
-<!DOCTYPE window SYSTEM "chrome://communicator-platform/locale/pref/platformPrefOverlay.dtd">  
+<!DOCTYPE window [
+<!ENTITY % brandDTD SYSTEM "chrome://global/locale/brand.dtd">
+%brandDTD;
+<!ENTITY % platformPrefOverlayDTD SYSTEM "chrome://communicator-platform/locale/pref/platformPrefOverlay.dtd">  
+%platformPrefOverlayDTD;
+]>
   
 <overlay id="platformPreferencesOverlay"
          xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
@@ -36,6 +42,172 @@
       </treerow>
     </treeitem>  
   </treechildren>
+
+  <!-- Add "set default browser" button to Navigator panel. -->
+  <hbox id="pref-nav-platform-extensions">
+    <groupbox flex="1">
+      <caption label="&defaultBrowserGroup.label;"/>
+      <deck id="defaultBrowserDeck" flex="1">
+        <!-- We use a deck for the three cases:
+             o If not already the default, then use child 0 (with button enabled)
+             o If already the default, then use child 1 (with disabled button).
+             o If the user has already pushed the "Set As Default" button, then use child 2
+               (with different text and a disabled button).
+        -->
+        <vbox flex="1">
+          <description>&makeDefaultText;</description>
+          <hbox>
+            <spacer flex="1"/>
+            <button label="&defaultBrowserButton.label;"
+                    oncommand="makeDefault()"/>
+            <spacer flex="1"/>
+          </hbox>
+        </vbox>
+        <vbox flex="1">
+          <description>&alreadyDefaultText;</description>
+          <hbox>
+            <spacer flex="1"/>
+            <button label="&defaultBrowserButton.label;"
+                    autostretch="never"
+                    disabled="true"/>
+            <spacer flex="1"/>
+          </hbox>
+        </vbox>
+        <vbox flex="1">
+          <description pack="start">&defaultPendingText;</description>
+          <hbox>
+            <spacer flex="1"/>
+            <button label="&defaultBrowserButton.label;"
+                    autostretch="never"
+                    disabled="true"/>
+            <spacer flex="1"/>
+          </hbox>
+        </vbox>
+      </deck>
+    </groupbox>
+  </hbox>
+  <script type="application/x-javascript">
+    <![CDATA[
+      // This overlay is pulled into multiple places.  We only want this
+      // code to kick in when we're overlaid onto pref-navigator.xul.  And,
+      // we don't want to conflict with code within other files.  So,
+      // we use anonymous functions throughout.
+      if (document.documentElement.id == "pref-navigator") {
+        // "Install" the SetFields function.  This will be
+        // called whenever the Navigator pane is shown.
+        window.SetFields = function( pageData ) {
+          // Determine if we have been selected as the default browser
+          // already, and enable/disable the "Set As Default" button
+          // accordingly.
+
+          // We store our state info in the same place as the code in
+          // pref-winhooks.js uses so that this panel and the 
+          // Advanced/System panel are kept in synch.
+          if (!("winHooks" in parent)) {
+            // Neither the Advanced/System panel nor this panel has
+            // appeared.  Initialize the state information.
+            parent.winHooks = new Object;
+
+            // Get winhooks service.
+            parent.winHooks.winhooks = Components.classes[ "@mozilla.org/winhooks;1" ]
+                                                 .getService( Components.interfaces.nsIWindowsHooks );
+          
+            // Extract current settings (these are what the user has checked on
+            // the Advanced/System panel).
+            parent.winHooks.prefs = parent.winHooks.winhooks.settings;
+          }
+
+          // Ensure our additional state info is set.
+          if (!("makeDefaultPending" in parent.winHooks)) {
+            parent.winHooks.makeDefaultPending = false;
+          }
+          
+          // Figure out which <deck> child should be activated.
+          // Default is child 0 (with button enabled).
+          var deckState = 0;
+          // Start by checking http/https/ftp and html/xhtml/xml.
+          var prefs = parent.winHooks.prefs;
+          if (prefs.isHandlingHTTP  &&
+              prefs.isHandlingHTTPS &&
+              prefs.isHandlingFTP   &&
+              prefs.isHandlingHTML  &&
+              prefs.isHandlingXHTML &&
+              prefs.isHandlingXML) {
+            // The user *wants* us to be the default, apparently.  This means
+            // that the deck *might* be in one of the other two states (with
+            // button disabled), depending on whether the registry matches.
+            // We test the registry settings using a scratch copy of the
+            // settings because we don't care about some of them, but we
+            // don't want to mess up the user's choices from the
+            // Advanced/System panel.
+            var testSettings = parent.winHooks.winhooks.settings;
+            // Test that these are set.
+            testSettings.isHandlingHTTP  = true;
+            testSettings.isHandlingHTTPS = true;
+            testSettings.isHandlingFTP   = true;
+            testSettings.isHandlingHTML  = true;
+            testSettings.isHandlingXHTML = true;
+            testSettings.isHandlingXML   = true;
+            // Ignore the rest.
+            testSettings.isHandlingCHROME = false;
+            testSettings.isHandlingGOPHER = false;
+            testSettings.isHandlingJPEG   = false;
+            testSettings.isHandlingGIF    = false;
+            testSettings.isHandlingMNG    = false;
+            testSettings.isHandlingPNG    = false;
+            testSettings.isHandlingBMP    = false;
+            testSettings.isHandlingICO    = false;
+            testSettings.isHandlingXUL    = false;
+            // Now test whether the registry matches that.
+            if ( testSettings.registryMatches ) {
+              // Disable the button.  The only remaining question
+              // is what text appears alongside it.  That will
+              // vary depending on whether the user has pressed
+              // the "Set As Default" button already.
+              if ( parent.winHooks.makeDefaultPending ) {
+                deckState = 2;
+              } else {
+                deckState = 1;
+              }
+            }
+          }
+    
+          // Now, switch the deck to the appropriate state.
+          document.getElementById("defaultBrowserDeck").selectedIndex = deckState;
+
+          // Return true to tell caller to process the standard pref fields.
+          return true;
+        }
+        // "Install" the makeDefault function;
+        window.makeDefault = function() {
+          // Extract current settings (these are what the
+          // user has checked on the Advanced/System panel).
+          var settings = parent.winHooks.prefs;
+
+          // Turn on all "default browser" settings.
+          settings.isHandlingHTTP  = true;
+          settings.isHandlingHTTPS = true;
+          settings.isHandlingFTP   = true;
+          settings.isHandlingHTML  = true;
+          settings.isHandlingXHTML = true;
+          settings.isHandlingXML   = true;
+
+          // Register callback that will be called when and if the
+          // user presses the OK button.
+          parent.hPrefWindow.registerOKCallbackFunc( function () {
+            // Apply the settings.
+            parent.winHooks.winhooks.settings = parent.winHooks.prefs;
+          } );
+
+          // Flip the deck to the "make-default pending" state.
+          document.getElementById("defaultBrowserDeck").selectedIndex = 2;
+
+          // Remember this state when next displayed.
+          parent.winHooks.makeDefaultPending = true;
+        }
+      }
+    ]]>
+  </script>
          
 </overlay>
 
diff --git a/xpfe/components/winhooks/nsIWindowsHooks.idl b/xpfe/components/winhooks/nsIWindowsHooks.idl
index c4f693cba0f6..81208481cdec 100644
--- a/xpfe/components/winhooks/nsIWindowsHooks.idl
+++ b/xpfe/components/winhooks/nsIWindowsHooks.idl
@@ -143,6 +143,12 @@ interface nsIWindowsHooksSettings : nsISupports {
     // appears on startup if there is a mismatch
     // between registry and these settings.
     attribute boolean showDialog;
+
+    // Registry mismatch indicator.
+    // This is true if the Win32 registry is
+    // currently set to match the preferences
+    // in this object.
+    readonly attribute boolean registryMatches;
 };
 
 /* nsIWindowsHooks
diff --git a/xpfe/components/winhooks/nsWindowsHooks.cpp b/xpfe/components/winhooks/nsWindowsHooks.cpp
index caab669a640b..a4accf821122 100644
--- a/xpfe/components/winhooks/nsWindowsHooks.cpp
+++ b/xpfe/components/winhooks/nsWindowsHooks.cpp
@@ -291,6 +291,48 @@ static PRBool isAccessRestricted() {
 
 
 
+// Implementation of method that checks whether the settings match what's in the
+// Windows registry.
+NS_IMETHODIMP
+nsWindowsHooksSettings::GetRegistryMatches( PRBool *_retval ) {
+    NS_ENSURE_ARG( _retval );
+    *_retval = PR_TRUE;
+    // Test registry for all selected attributes.
+    if ( misMatch( mHandleHTTP,   http )
+         ||
+         misMatch( mHandleHTTPS,  https )
+         ||
+         misMatch( mHandleFTP,    ftp )
+         ||
+         misMatch( mHandleCHROME, chrome )
+         ||
+         misMatch( mHandleGOPHER, gopher )
+         ||
+         misMatch( mHandleHTML,   mozillaMarkup )
+         ||
+         misMatch( mHandleJPEG,   jpg )
+         ||
+         misMatch( mHandleGIF,    gif )
+         ||
+         misMatch( mHandlePNG,    png )
+         ||
+         misMatch( mHandleMNG,    mng )
+         ||
+         misMatch( mHandleBMP,    bmp )
+         ||
+         misMatch( mHandleICO,    ico )
+         ||
+         misMatch( mHandleXML,    xml )
+         ||
+         misMatch( mHandleXHTML,  xhtml )
+         ||
+         misMatch( mHandleXUL,    xul ) ) {
+        // Registry is out of synch.
+        *_retval = PR_FALSE;
+    }
+    return NS_OK;
+}
+
 // Implementation of method that checks settings versus registry and prompts user
 // if out of synch.
 NS_IMETHODIMP
@@ -357,35 +399,9 @@ nsWindowsHooks::CheckSettings( nsIDOMWindowInternal *aParent,
         // First, make sure the user cares.
         if ( settings->mShowDialog || installing ) {
             // Look at registry setting for all things that are set.
-            if ( misMatch( settings->mHandleHTTP,   http )
-                 ||
-                 misMatch( settings->mHandleHTTPS,  https )
-                 ||
-                 misMatch( settings->mHandleFTP,    ftp )
-                 ||
-                 misMatch( settings->mHandleCHROME, chrome )
-                 ||
-                 misMatch( settings->mHandleGOPHER, gopher )
-                 ||
-                 misMatch( settings->mHandleHTML,   mozillaMarkup )
-                 ||
-                 misMatch( settings->mHandleJPEG,   jpg )
-                 ||
-                 misMatch( settings->mHandleGIF,    gif )
-                 ||
-                 misMatch( settings->mHandlePNG,    png )
-                 ||
-                 misMatch( settings->mHandleMNG,    mng )
-                 ||
-                 misMatch( settings->mHandleBMP,    bmp )
-                 ||
-                 misMatch( settings->mHandleICO,    ico )
-                 ||
-                 misMatch( settings->mHandleXML,    xml )
-                 ||
-                 misMatch( settings->mHandleXHTML,  xhtml )
-                 ||
-                 misMatch( settings->mHandleXUL,    xul )) {
+            PRBool matches = PR_TRUE;
+            settings->GetRegistryMatches( &matches );
+            if ( !matches ) {
                 // Need to prompt user.
                 // First:
                 //   o We need the common dialog service to show the dialog.
diff --git a/xpfe/global/resources/content/nsWidgetStateManager.js b/xpfe/global/resources/content/nsWidgetStateManager.js
index b933e9cd3b5b..35f826de3cce 100644
--- a/xpfe/global/resources/content/nsWidgetStateManager.js
+++ b/xpfe/global/resources/content/nsWidgetStateManager.js
@@ -163,8 +163,12 @@ nsWidgetStateManager.prototype =
           var pageData = this.dataManager.getPageData( aPageTag );
           if( 'SetFields' in this.contentArea )
             {
-              this.contentArea.SetFields( pageData );
-              return;
+              if ( !this.contentArea.SetFields( pageData ) )
+              {
+                // If the function returns false (or null/undefined) then it
+                // doesn't want *us* to process the page data.
+                return;
+              }
             }
 
           for( var elementID in pageData )