Right click somewhere in the top allows to show/hide the menu bar.
One line per message (table view)
Somewhere in the top right area above the messages is a Quick Filter (not to be confused with the Filter these messages input). Right to the Quick Filter is an icon that allows to choose between Table view and Cards view.
Settings menu
The Settings menu is «hidden behind» the three horizontal bars close to the top right corner of the Thunderbird GUI, not, as possibly expected, under the Tools menu.
When adding a gmail-based account, only the email address and name need to be entered, but not the password.
Thunderbird will then open a Google sign-in page where the password can be entered.
Writing messages in or without HTML format
In left pane: Right mouse click on account -> Settings -> Composition & Addressing -> (un)tick Compose messages in HTML format.
Don't automatically mark messages as read
Settings -> General -> Reading & Display -> [] Automatically mark messages as read
Turn off notifications when mail arrives
(top right) -> Settings -> General -> Incoming Mails -> [] Show an alert / [] Use the system notification / [] Play a sound
Never send a return receipt
Settings -> Account Settings -> Return Receipts -> Use my global return receipt preference for this account -> Global preferences -> [x] Never send a return receipt.
or…
Settings -> Account Settings -> Return Receipts -> [x] Use my global returen receipt preferences for this account -> Global Preferences -> [x] Never send a return receipt.
Embedded images are not sent
Observation:
Images that are inserted (not attached) with Insert -> Image… are not sent to the recipient
The account's settings for Composition & Addressing have the item Compose messages in HTML format checked
Solution: Set value of of Sending Format in Thunderbird Settings under Composition to Automatic.
As it is an unofficial plugin, I had to download it with wget http://globs.org/file/exteditor_v100.xpi. (The «normal» install wouldn't work).
Installing it
Tools -> Add-ons -> Extensions. Click on «tools» symbol (screwdriver and wrench) in the top. -> Install Add-on From File.
Restart Thunderbird.
Modify preferences (still in Tools -> Add-ons -> Extensions): Set Text Editor = gvim -f. (-f is for no fork).
Adding the External Editor button to the toolbar: File -> New -> Message, then View -> Toolbars -> Customize..., then drag External Editor to toolbar of new message.
Using it
Messages can now be edited with ctrl-e.
Adding a signature
In left most pane, click on email address (right above Inbox) for which a signature is to be added.