Using the layout submenu in the View menu, I have set Tbird to display only the folder list and the currently selected  folder displaying the list of messages, which is about as bare bones as it gets.  Pressing enter on a message opens the body, esc to return to the list.  In the Layout sub-menu I only have the Vertical view, and folder list items checked.  The tab key just loops arount between the tab control of the currently selected folder, the selected folder in the folder treeview and the currently select message in the list window.  The listed messages details are accessed using the table navigation keys Control Alt Arrows.  It’s a while since I set it up, there may be other settings I made to get it to this state, which I did to make it easy for a friend of mine to use.  NVDA  usually says unknown when it encounters a control or unlabelled icon it can’t interpret.  I get this after pressing enter on a message in the list, when the message body opens but I’ve not experienced any difficulties using it.

For completeness, Control n to compose a new message, Control enter to send it. 

Cheers

Chris