diff --git a/mail/components/prefwindow/content/pref-mailnews.xul b/mail/components/prefwindow/content/pref-mailnews.xul
index 82d3766d70ed..5a73851d1463 100644
--- a/mail/components/prefwindow/content/pref-mailnews.xul
+++ b/mail/components/prefwindow/content/pref-mailnews.xul
@@ -39,7 +39,7 @@
   <script type="application/x-javascript">
   <![CDATA[
 
-    var _elementIDs = ["mailnewsStartPageEnabled", "mailnewsStartPageUrl", "mailPaneConfig", "mailRememberLastMsg",
+    var _elementIDs = ["mailnewsStartPageEnabled", "mailnewsStartPageUrl", "mailPaneConfig", "mailnewsDoubleClick2NewWindow", "mailRememberLastMsg",
                        "mailnewsSoundFileUrl", "newMailNotification", "newMailNotificationType",
                         "newMailNotificationAlert"];
   ]]>
@@ -60,6 +60,8 @@
     
   </groupbox>
   
+  <hbox flex="1">
+
   <groupbox>
     <caption label="&windowSettings.label;"/>
     <label value="&selectWindowLayout.label;" accesskey="&selectWindowLayout.accesskey;" control="mailPaneConfig"/>
@@ -71,6 +73,21 @@
     </hbox> 
   </groupbox>
 
+  <spring flex="1"/>
+
+  <groupbox>
+    <caption label="&windowBehavior.label;"/>
+    <label value="&reuseExp.label;"/>
+    <hbox class="indent" align="center">
+      <radiogroup id="mailnewsDoubleClick2NewWindow" preftype="bool" prefstring="mailnews.reuse_message_window" orient="vertical">
+        <radio value="false" label="&reuseExpRadio0.label;" accesskey="&reuseExpRadio0.accesskey;" id="new"/>
+        <radio value="true" label="&reuseExpRadio1.label;" accesskey="&reuseExpRadio1.accesskey;" id="existing"/>
+      </radiogroup>
+    </hbox>
+  </groupbox>
+
+  </hbox>
+
   <groupbox>
     <caption label="&messengerStartPage.label;"/>
       <hbox align="center">
@@ -105,7 +122,7 @@
          oncommand="PlaySoundCheck()"/>
 
     <hbox>
-      <hbox align="center" class="indent">
+      <hbox align="center">
         <radiogroup id="newMailNotificationType" prefstring="mail.biff.play_sound.type" orient="vertical">
           <radio class="iconic" value="0" oncommand="PlaySoundCheck()" label="&systemsound.label;" id="system"/>
           <radio class="iconic" value="1" oncommand="PlaySoundCheck()" label="&customsound.label;" id="custom"/>
diff --git a/mail/components/prefwindow/locale/mailPrefsOverlay.dtd b/mail/components/prefwindow/locale/mailPrefsOverlay.dtd
index 66cb5969e207..38b35b02469c 100644
--- a/mail/components/prefwindow/locale/mailPrefsOverlay.dtd
+++ b/mail/components/prefwindow/locale/mailPrefsOverlay.dtd
@@ -27,3 +27,4 @@
 <!ENTITY extensions.label "Extensions">
 <!ENTITY fonts.label "Fonts">
 <!ENTITY privacy.label "Privacy">
+
diff --git a/mail/components/prefwindow/locale/pref-mailnews.dtd b/mail/components/prefwindow/locale/pref-mailnews.dtd
index a2f5f6e5eff5..d6f582125b32 100644
--- a/mail/components/prefwindow/locale/pref-mailnews.dtd
+++ b/mail/components/prefwindow/locale/pref-mailnews.dtd
@@ -18,6 +18,15 @@
 <!ENTITY warnOnSendAccelKey.label         "Confirm when using keyboard shortcut to send message">
 <!ENTITY warnOnSendAccelKey.accesskey     "o">
 
-<!ENTITY windowSettings.label             "Windows">
+<!ENTITY windowSettings.label             "Window Configuration">
 <!ENTITY selectWindowLayout.label         "Select the window layout you prefer for Mail.">
-<!ENTITY selectWindowLayout.accesskey     "S">
\ No newline at end of file
+<!ENTITY selectWindowLayout.accesskey     "S">
+
+<!ENTITY windowBehavior.label             "Message Window Behavior">
+<!ENTITY reuseExp.label                   "Open new messages in:">
+<!ENTITY reuseExpRadio0.label             "A new message window">
+<!ENTITY reuseExpRadio0.accesskey         "n">
+<!ENTITY reuseExpRadio1.label             "An existing message window">
+<!ENTITY reuseExpRadio1.accesskey         "e">
+<!ENTITY reuseExpRadio2.label             "A tab in an existing message window">
+<!ENTITY reuseExpRadio2.accesskey         "t">
\ No newline at end of file