Good write up on the Mozilla blog about updating the compose window and some of the things changing and being experimented with (and why).

We're currently exploring ways to relieve the pressure, and move on to a saner design for us. I've been working on an experiment that aims at tackling these pain points. It basically consists in replacing the standard XUL compose window with a tab written in XHTML that integrates CKEditor, and doing all the composition process in JS, upto the actual send that takes places in C++.