Besides the moz support pages that Tb guides you to, accessing the
m'zine kb has useful info, but sometimes old.
This kb page describes the # meaning of the reply header setting and how
to get to the config editor:
http://kb.mozillazine.org/Reply_header_settings
For one type of navigation purposes, that kb Tb front page is:
http://kb.mozillazine.org/Category:Thunderbird - Articles relating to
the Thunderbird e-mail and newsgroup client. Each page typically has a
list of subcategories at the top, and a list of articles in the current
category/subcategory at the bottom of the page. If you can't find what
you're looking for by navigating these categories, try entering some
keywords in the search field to the right. New users should read Getting
started with Thunderbird and How to keep Thunderbird working smoothly.
If you only use Tb help, you are only guided to the moz support pages
which start here and are quite useful, but 'slight' (ignore) the m'zine
kb usefulness.
https://support.mozilla.org/en-US/products/thunderbird
--
Mike Easter