When reading an email, the ESC key can be used to close the window. This doesn't work when you're composing an email. Is there any way to do this? This happened in T-Bird 2.x, as well, but I'm looking for a 3.0 solution.
use ALT + F4 instead. works for both, messages opened in new windows and indeed the 'write mail' window.
if you want a single key, you can of course remap ALT + F4 (globally) with Keymapper to any other key. however, i wouldn't recommend ESC for various reasons, i'm using Pause instead (which has an unwanted impact on NumLock but there's a work around).
Keymapper is open source freeware.