https://support.mozilla.org/en-US/products/thunderbird#search

when you do your search refine it under the text box to only help articles if that is what you are looking for.

I have been to that site. It is not informative. Seems to be incomplete. Hard to understand the lingo for someone who is relatively new and struggling with a problem. Needs a beginner's guide and a TOC using words that are themselves explained and related to the words used in the application itself. A minor example: using this reply form could use a little guidance about "Common Response". Also how to find this dialogue afterwards.