Comment 50 for bug 334344

You don't. It has been moved to target Thunderbird 3.2 according to comment 23 above: https://bugzilla.mozilla.org/show_bug.cgi?id=478463#c23

However, there is an extension that works with libnotify and the indicator applet. See here for more details: http://ubublogger.wordpress.com/2010/06/28/new-version-of-libnotify-for-mozilla-0-2-stable/