Open a URL within Thunderbird's Mail Start Page

Here is how to load a URL from an extension in Thunderbird, into the Mail Start Page area:

var url = "http://www.mikechambers.com";
var browser = document.getElementById("messagepane");
browser.loadURI(url, browser.currentURL, null);

That will allow you to change the current document loaded in the Mail Start Page (email reading) area of Thunderbird.