Just installed Thunderbird 38.0.1 (June 23, 2015) Identity Chooser 1.8.8 add on was not flagged as being incompatible; however it 'remembers' the address(es) selected from the previously crafted email and auto inserts it in the address bar for the next crafted reply. After disabling Identity Chooser 1.8.8, the problem was fixed. It is likely an incompatibility with the add on.
There is a newer version of Identity Chooser [version 1.9.2]. This version works with TB 38.0.1. The recommended fix would be to flag any older versions as incompatible during the TB install.