--- a/browser/base/content/test/social/browser_social_chatwindow.js
+++ b/browser/base/content/test/social/browser_social_chatwindow.js
@@ -474,18 +474,48 @@ var tests = {
         openChat(Social.provider, function() {
           ok(secondWindow.SocialChatBar.hasChats, "second window now has chats");
           is(window.SocialChatBar.chatbar.childElementCount, 1, "first window still has 1 chat");
           window.SocialChatBar.chatbar.removeAll();
           // now open another chat - it should still open in the second.
           openChat(Social.provider, function() {
             ok(!window.SocialChatBar.hasChats, "first window has no chats");
             ok(secondWindow.SocialChatBar.hasChats, "second window has a chat");
-            secondWindow.close();
-            next();
+
+            // focus the first window, and open yet another chat - it
+            // should open in the first window.
+            waitForFocus(function() {
+              openChat(Social.provider, function() {
+                ok(window.SocialChatBar.hasChats, "first window has chats");
+                window.SocialChatBar.chatbar.removeAll();
+                ok(!window.SocialChatBar.hasChats, "first window has no chats");
+
+                let privateWindow = OpenBrowserWindow({private: true});
+                privateWindow.addEventListener("load", function loadListener() {
+                  privateWindow.removeEventListener("load", loadListener);
+
+                  // open a last chat - the focused window can't accept
+                  // chats (it's a private window), so the chat should open
+                  // in the window that was selected before. This is known
+                  // to be broken on Linux.
+                  openChat(Social.provider, function() {
+                    let os = Services.appinfo.OS;
+                    const BROKEN_WM_Z_ORDER = os != "WINNT" && os != "Darwin";
+                    let fn = BROKEN_WM_Z_ORDER ? todo : ok;
+                    fn(window.SocialChatBar.hasChats, "first window has a chat");
+                    window.SocialChatBar.chatbar.removeAll();
+
+                    privateWindow.close();
+                    secondWindow.close();
+                    next();
+                  });
+                });
+              });
+            });
+            window.focus();
           });
         });
       })
     });
   },
   testMultipleProviderChat: function(next) {
     // while pref'd off, we need to set the worker mode to multiple providers
     setWorkerMode(true, function() {

--- a/toolkit/components/social/MozSocialAPI.jsm
+++ b/toolkit/components/social/MozSocialAPI.jsm
@@ -270,22 +270,33 @@ function isWindowGoodForChats(win) {
 }
 
 function findChromeWindowForChats(preferredWindow) {
   if (preferredWindow && isWindowGoodForChats(preferredWindow))
     return preferredWindow;
   // no good - we just use the "most recent" browser window which can host
   // chats (we used to try and "group" all chats in the same browser window,
   // but that didn't work out so well - see bug 835111
+
+  // Try first the most recent window as getMostRecentWindow works
+  // even on platforms where getZOrderDOMWindowEnumerator is broken
+  // (ie. Linux).  This will handle most cases, but won't work if the
+  // foreground window is a popup.
+
+  let mostRecent = Services.wm.getMostRecentWindow("navigator:browser");
+  if (isWindowGoodForChats(mostRecent))
+    return mostRecent;
+
   let topMost, enumerator;
-  // *sigh* - getZOrderDOMWindowEnumerator is broken everywhere other than
-  // Windows.  We use BROKEN_WM_Z_ORDER as that is what the c++ code uses
+  // *sigh* - getZOrderDOMWindowEnumerator is broken except on Mac and
+  // Windows.  We use BROKEN_WM_Z_ORDER as that is what some other code uses
   // and a few bugs recommend searching mxr for this symbol to identify the
   // workarounds - we want this code to be hit in such searches.
-  const BROKEN_WM_Z_ORDER = Services.appinfo.OS != "WINNT";
+  let os = Services.appinfo.OS;
+  const BROKEN_WM_Z_ORDER = os != "WINNT" && os != "Darwin";
   if (BROKEN_WM_Z_ORDER) {
     // this is oldest to newest and no way to change the order.
     enumerator = Services.wm.getEnumerator("navigator:browser");
   } else {
     // here we explicitly ask for bottom-to-top so we can use the same logic
     // where BROKEN_WM_Z_ORDER is true.
     enumerator = Services.wm.getZOrderDOMWindowEnumerator("navigator:browser", false);
   }