It appears the Identity Chooser add-on (version 1.8.2) and Thunderbird 20 Beta (to be released as Thunderbird 17.0.5 on April 2nd) do not play well together. I discovered I could only reply to messages. I was no longer able to compose, forward or ‘edit as new’ (though never used that feature) a message. Also could not change the sending account in a message I was replying. After disabling the add-on, everything appears to be working as it should. After checking on AMO and seeing there was not a newer version on the development channel, I contacted the developer letting him know of the issues. I will post an update once I get more information.