Bug #234541 --> On Mac OS X, clicking on a mailto URL in Firefox or Safari when Thunderbird is already open, fails to open

a compose window. Instead, you get an error failing to find navigator.xul.

Change the value of browser.chrome to point to the messengercompose chrome.

mail/app/profile/all-thunderbird.js

@@ -43,6 +43,7 @@ pref("general.skins.selectedSkin", "classic/1.0");
 pref("app.extensions.version", "1.0");
 
 #ifdef XP_MACOSX
+pref("browser.chromeURL", "chrome://messenger/content/messengercompose/messengercompose.xul");
 pref("mail.biff.animate_dock_icon", false);
 #endif