Re:What is new? (Score 1)

Can I modify the From address by simply tyoing it in the From: line, without creating an account/identity for it?

Yes, but not easily. Have a read of the Multiple Identity Support page (this is also going into Mozilla 1.7).

You will need to edit prefs.js for every address you want to add. The document does suggest that a forthcoming version of Thunderbird will have a UI for this.