What follows is, as the person calls it, a "Thunderbird cheatsheet", 
containing basic instructions to deal with the Thunderbird's main functions.
These instructions were written by Brian V.,  a member of the NVDA's lists 
and Win 10 for Screen Reader Users' forum.

Thunderbird e-mail client
IMPORTANT:  For those using Thunderbird with a screen reader I strongly 
suggest installing the Quick Folder Key Navigation extension.  You will only 
need to install this extension once for any instance of Thunderbird you 
might be using.  This allows you to jump from folder in the folder tree 
using first letter navigation to move between them.  Without it, this does 
not always work.   You will only need to install this extension once for any 
instance of Thunderbird you might be using.  If you are using JAWS or NVDA, 
searching for this add on within Thunderbird is quite simple:
         Activate the Tools Menu (ALT+T), add-ons option (followed by A) 
and tab 3 times, at which point you should be on the Extensions Pane.  If 
you happen to land on any other pane then use Down Arrow (or Up Arrow) until 
you hear Extensions.
         Tab 2 times, which lands you in the search all add-ons edit box.
         Then type in Quick Folder Key Navigation, hit enter, and wait a 
few seconds for the search to complete.
         Tab 5 times more gain focus on the list of results.  Then hit Down 
Arrow to start navigating the list.  Quick Folder Key Navigation should be 
the first result.
         Tab 2 times to land on the Install button then hit enter.
         After the install completes you need to exit Thunderbird and start 
it up again for this extension to become active
Upon opening Thunderbird, you will be in the folder tree sitting on whatever 
folder you had open the last time you closed it.  You will still have to hit 
TAB until you reach the folder tree view.
When in the folder tree view you can use first letter navigation to move 
around (if you installed the extension noted above).
When you are sitting on a folder in the folder tree view hitting TAB will 
take you to the message list.  You know youve arrived when you hear, List 
View.  Use up and/or down arrow to navigate through the list of messages. 
[Depending on the screen reader you usually have to hit TAB either once or 
twice.]
If you are using threaded view, and hear JAWS announce Level zero, 
collapsed, use the right arrow to expand the thread to show all its 
individual messages.  To collapse the thread, get back to the first message 
at level zero and hit the left arrow.
When you land on the message you want to read, hit Enter to open it in a 
dedicated reading tab.
You will use the usual reading commands to read through the message that you 
use in any other program.
Thunderbird Shortcuts
Hit CTRL+W to close a reading tab and go back to the message list for the 
folder you had been viewing.
Hit SHIFT+TAB (once or twice) or F6 to move back to the folder tree from the 
message list for any folder.  This must be done after you delete all 
messages in a given folder as well.
To select messages in the message list view:
            Once you are in the message list view, and have a specific 
single message selected:
1.      Holding SHIFT, while at the same time hitting either Up or Down 
Arrow will continue selecting messages in the direction you are moving.
2.      Holding SHIFT, then hitting PgUp or PgDown will select all messages 
above or below the currently selected message, respectively, stopping with 
the first/last one currently displayed in the message list view.
You can use CTRL+A to select all messages, including ones currently not 
visible in the message list view.  Unless youre certain about doing 
whatever you intend to do with ALL messages, use this option with great 
caution.
To delete messages:
1.      If you are in the message list view, select the message or messages 
you wish to delete then hit the Delete key.  This will move them to the 
Trash folder and none will be opened for reading if you use this technique.
2.      If you are reading a message and want to delete it, hit the Delete 
key and it will delete that message and automatically open the next message 
in the folder for reading.
To reply to a message youre reading:  CTRL+R opens the Write window with 
the subject filled in with Re: prefixing it and the text of the message 
youre replying to quoted.  Your cursor will be sitting right after the 
quoted material waiting for you to type.  If  youre trying to do a Reply 
All thats CTRL+Shift+R
To create a new message, regardless of what youre doing in Thunderbird, hit 
CTRL+N.  This opens the Write window waiting for you to fill in your 
addressees in the To: edit box.  As you hit Enter after filling in an 
address or choosing one from the dropdown list of previous addresses another 
edit box of the same type opens.  If you need to change from To: to CC: 
or BCC: then hit SHIFT+TAB, use the up/down arrow key to find what you 
want, then hit TAB again to go to the edit box for the address.
         To add attachments to an outgoing message, hit CTRL+SHIFT+A to 
bring up the Attach Files dialog.  It is very similar to a Save dialog 
except it works in reverse.  You will need to navigate to the location of 
the file you want to attach, select it, then either tab to the Open button 
or hit ALT+O to attach the file to the message.
To send a message, whether its a reply or a new message, hit CTRL+ENTER 
when youve finished composing.  It will send immediately and close the 
Write window, taking you back to wherever you had been before hitting the 
reply or new keyboard shortcut.
To get to attachments on an incoming message hit ALT+M, H  [Opens the 
Message Menu and selects the Attachments option]
Creating Folders in Thunderbird
            You can create folders within Thunderbird to organize your 
e-mail.  The concept is the same as in Windows File Explorer, and the tree 
view will be the starting point.
1.      In the tree view navigate to the folder or account (if you want the 
folder directly under the account like Inbox, Sent, etc., typically are) 
under which you want your new folder nested.  In most cases this will be 
your e-mail account itself, unless you are creating subfolders within other 
existing folders.
2.      Hit ALT+F,N,F  File Menu, New Option, Folder Option  after which 
the New Folder dialog opens and youll be in the edit box in which youll 
type the new folders name.
3.      Tab to the Create Folder button then hit enter, or hit ALT+R.  Your 
new folder has been created.
Moving Messages Between Folders on Thunderbird
1.      Select the message or messages you wish to move.
2.      Bring up the context menu with the applications/menu key or 
Shift+F10, then hit M [Move]
3.      You will be presented with a submenu that will vary somewhat 
depending on your folder structure, you have to drill down through the 
submenu to locate the folder where you want the selected messages to be 
moved.
4.      Once you have focus on that folder, hit Enter and the message(s) 
will be moved there.