diff --git a/calendar/base/content/preferences/connection.js b/calendar/base/content/preferences/connection.js
--- a/calendar/base/content/preferences/connection.js
+++ b/calendar/base/content/preferences/connection.js
@@ -69,17 +69,22 @@ var gConnectionsDialog = {
       }
     }
     
     var noProxiesPref = document.getElementById("network.proxy.no_proxies_on");
     noProxiesPref.value = noProxiesPref.value.replace(/[;]/g,',');
     
     return true;
   },
-  
+ 
+  checkForSystemProxy: function ()
+  {
+    if ("@mozilla.org/system-proxy-settings;1" in Components.classes)
+      document.getElementById("systemPref").removeAttribute("hidden");
+  },
 
   /**
    * Handler function to be called when the network.proxy.type preference has
    * changed while the connection preferences dialog is open.
    */
   proxyTypeChanged: function gCD_proxyTypeChanged() {
     var proxyTypePref = document.getElementById("network.proxy.type");
     
diff --git a/calendar/base/content/preferences/connection.xul b/calendar/base/content/preferences/connection.xul
--- a/calendar/base/content/preferences/connection.xul
+++ b/calendar/base/content/preferences/connection.xul
@@ -40,16 +40,17 @@
 
 <?xml-stylesheet href="chrome://global/skin/"?>
 
 <prefwindow id="ConnectionsDialog" type="child"
             xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
             title="&connectionsDialog.title;"
             dlgbuttons="accept,cancel"
             onbeforeaccept="return gConnectionsDialog.beforeAccept();"
+            onload="gConnectionsDialog.checkForSystemProxy();"
 #ifdef XP_MACOSX
             style="width: &window.macWidth; !important;">
 #else
             style="width: &window.width; !important;">
 #endif
 
   <prefpane id="ConnectionsDialogPane">
 
@@ -88,16 +89,17 @@
 
     <groupbox>
       <caption label="&proxyTitle.label;"/>
 
       <radiogroup id="networkProxyType" preference="network.proxy.type"
                   onsyncfrompreference="return gConnectionsDialog.readProxyType();">
         <radio value="0" label="&directTypeRadio.label;" accesskey="&directTypeRadio.accesskey;"/>
         <radio value="4" label="&WPADTypeRadio.label;" accesskey="&WPADTypeRadio.accesskey;"/>
+        <radio value="5" label="&systemTypeRadio.label;" accesskey="&systemTypeRadio.accesskey;" id="systemPref" hidden="true"/>
         <radio value="1" label="&manualTypeRadio.label;" accesskey="&manualTypeRadio.accesskey;"/>
         <grid class="indent" flex="1">
           <columns>
             <column/>
             <column flex="1"/>
           </columns>
           <rows>
             <row align="center">
diff --git a/calendar/locales/en-US/chrome/calendar/preferences/connection.dtd b/calendar/locales/en-US/chrome/calendar/preferences/connection.dtd
--- a/calendar/locales/en-US/chrome/calendar/preferences/connection.dtd
+++ b/calendar/locales/en-US/chrome/calendar/preferences/connection.dtd
@@ -43,16 +43,18 @@
 <!ENTITY  window.width                  "37em">
 <!ENTITY  window.macWidth               "39em">
 
 <!ENTITY  proxyTitle.label              "Configure Proxies to Access the Internet">
 <!ENTITY  directTypeRadio.label         "Direct connection to the Internet">
 <!ENTITY  directTypeRadio.accesskey     "d">
 <!ENTITY  WPADTypeRadio.label           "Auto-detect proxy settings for this network">
 <!ENTITY  WPADTypeRadio.accesskey       "w">
+<!ENTITY  systemTypeRadio.label         "Use system proxy settings">
+<!ENTITY  systemTypeRadio.accesskey     "u">
 <!ENTITY  manualTypeRadio.label         "Manual proxy configuration:">
 <!ENTITY  manualTypeRadio.accesskey     "m">
 <!ENTITY  autoTypeRadio.label           "Automatic proxy configuration URL:">
 <!ENTITY  autoTypeRadio.accesskey       "A">
 <!ENTITY  reload.label                  "Reload">
 <!ENTITY  reload.accesskey              "e">
 <!ENTITY  ftp.label                     "FTP Proxy:">
 <!ENTITY  ftp.accesskey                 "F">