Please try not to update to 20090120 TB nightlies…

Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20090120 Lightning/1.0pre Shredder/3.0b2pre

and other platforms as well, it is affected by bug 474389 ( Can’t focus or type in editor in mail compose window ) which was caused by bug 459443 ([FIX]Scripts are disabled when navigating away from designMode document).

This means you cannot compose any email yet, much less reply.

You can get the previous day’s nightlies here.