vim visual line mode-like selection navigation about tbkeys HOT 5 OPEN

Are you asking to configure something like the select multiple adjacent messages keyboard shortcut?

Select the first message of your intended selection, and then either Shift + Click on the last message of your selection, or hold Shift while using any of the following movement keys to expand your selection: ↓, ↑, Page Down, Page Up, End, or Home.

Thunderbird Keyboard shortcuts (search for adjacent)
http://support.mozilla.org/kb/keyboard-shortcuts-thunderbird

How to simulate or synthesize a keypress or a mouse click
http://forums.mozillazine.org/viewtopic.php?p=14949538#p14949538

There may be a cmd_ command that selects an adjacent message in the thread pane, but I don't think so.

window.goDoCommand('cmd_selectAll');

The cmd_selectAll command selects all messages in the thread pane.

from tbkeys.