Thunderbird on Mac
Internal
Installation
Homebrew
Needs cask support to be installed.
Then:
brew cask install thunderbird
Configuration
Local Mail Storage
IMAP Storage
/Users/<username>/Library/Thunderbird/Profiles/<profile-id>.default/ImapMail/<imap-account-name>/<IMAP-subfolders>
Local Directory
/Users/<username>/Library/Thunderbird/Profiles/<profile-id>.default/Mail/Local Folders
It had been tested that is possible to zip "Local Folders", move it under the Profiles directory of another installation and keep using it from there.
It also has been tested that the "Local Folders" can be moved on an arbitrary location on the filesystem and Thunderbird reconfigured to open it at Tools → Account Settings → Local Folders → Local Directory → Browse and set.
Cache
/Users/<username>/Library/Caches/Thunderbird
Cleanup
Locate the local e-mail storage for each account (Tools → Account Settings → Account → Server Settings → Local Directory) and write it down.
Remove each one of them. The entire /Users/<username>/Library/Thunderbird can be removed if Thunderbird is removed from the system.
Also remove /Users/<username>/Library/Caches/Thunderbird.
Uninstall Thunderbird: Open the Applications folder and click and drag the Thunderbird icon to Trash. Note that uninstalling it this way does NOT clean up the profiles stored in ~/Library/Thunderbird.