[senit] not SEN

Anybody know what I've done to make messages in Thunderbird openable 
ONLY "in a new window"- i.e. double clicking or single clicking the 
title in the message box no longer works?

I also find that if I save a message (and it goes into the drafts 
folder) I again have to open it in a new window and can no longer alter 
or add to the message when I try to continue with it.

I'm using  Thunderbird -  version 2.0.0.6 (20070728).   It tells me 
there are no further updates available (updates itself  in the background).

Thank you in advance for any suggestions.

Elizabeth