Thunderbird, for some reason I don't understand, has no implementation of true Forward and Backward buttons. The buttons they have only go to the next or previous UNREAD messages. There are few to no add-on buttons for that either, for the current v5.0 version of Thunderbird.
Instead of dealing with shoe-horning a v3 version of buttons into v5, I simply use the F (for forward) and B (for backward) buttons, while reading messages. This works great and keeps the desktop clutter, and multiple clickings, to a minimum.