diff --git a/mozilla/extensions/irc/xul/content/channels.js b/mozilla/extensions/irc/xul/content/channels.js
index cf76b2d783b..11b50efb79b 100644
--- a/mozilla/extensions/irc/xul/content/channels.js
+++ b/mozilla/extensions/irc/xul/content/channels.js
@@ -131,6 +131,10 @@ function onLoad()
             window[m] = client.mainWindow[m];
     }
 
+    // Set the <dialog>'s class so we can do platform-specific CSS.
+    var dialog = document.getElementById("chatzilla-window");
+    dialog.className = "platform-" + client.platform;
+
     var tree = document.getElementById("channelList");
     channelTreeBoxObject = tree.treeBoxObject;
 
diff --git a/mozilla/extensions/irc/xul/skin/channels.css b/mozilla/extensions/irc/xul/skin/channels.css
index e5fb6ca02b6..e4a553b9e6c 100644
--- a/mozilla/extensions/irc/xul/skin/channels.css
+++ b/mozilla/extensions/irc/xul/skin/channels.css
@@ -41,6 +41,10 @@
 	width: 12em;
 }
 
+.platform-Mac #rightPanel {
+	width: 18em;
+}
+
 #loadContainer {
 	margin: 0;
 }