This post is mainly of interest to Sunbird/Lightning extension developers. Due to the recently fixed bug 290747, the chrome:// URL of the Sunbird main window has changed. This was long overdue, since we don’t want to be shipping Sunbird-only code with Lightning.
For you as an extension developer, this means you will need to change the URL in your chrome.manifest when overlaying the Sunbird window. For the lazy, here a line to copy, paste and modify:
overlay chrome://sunbird/content/calendar.xul chrome://myext/content/myext.xul application={718e30fb-e89b-41dd-9da7-e25a45638b28}
If you use the Mozilla Build System to build your extensions (see this page for an intro on how to do that), then you should use this line in your jar.mn:
% overlay chrome://sunbird/content/calendar.xul chrome://myext/content/myext.xul application={718e30fb-e89b-41dd-9da7-e25a45638b28}
Of course this change is only needed now if you want to keep your extension compatible with the current nightlys, but at latest with the next version (1.0), you will need to take care.