Rated 5 out of 5 stars

Από τον/την Cocinero στις Αύγ. 11, 2010 · μόνιμος σύνδεσμος

(Please ignore my rating, I wasn't allowed to post without it. This is not a review, but rather a reply to raajsaab. Apparently, this isn't supported by this forum.)

@raajsaab: An extension named "XNote" provides the functionality you seek: https://addons.mozilla.org/de/thunderbird/addon/3093/
It's OK for Thunderbird version 2.0.0.*, I've been using it for a couple of months now on several machines and OSes.

There's also a version by another author for TB 3 based on the old one ( https://addons.mozilla.org/de/thunderbird/addon/64758/ ), but it has not yet been reviewed by Mozilla.
Also there is "XNote Plus" ( https://addons.mozilla.org/de/thunderbird/addon/5730/ ), which hasn't been reviewed either. I don't know the difference between the latter two, though.

Hope that helps.
Regards,
R.