+1

It would be nice if there was a mechanism in thunderbird that would replace the normal messagepane with a chrome or html based form based on the sender of maybe part of the subject.

than you could make it work for facebook, hyves or any other site..

Great idea. Could also work for Twitter direct messages.

Good idea, I'd use it if it was there.
Extra credit for being able to post the replay back to Facebook if the email address isn't available.

And I guess you might want similar functionality for other social sites.

I've tried tinkering with this idea today, and of the four steps above (identify the contact, replace the reply-to field, scrape the subject, and replace the subject field), the two scraping steps are trivial to do in TB3, but the replacement steps are impossible, AFAIK. If anyone knows how to do that (i.e., persistently change the header fields of a downloaded message), then it should be easy to finish up.