ReminderFox Requires Restart

ReminderFox displays and manages lists of reminders and tasks. ReminderFox uses easy-to-use lists with warnings/alarms to remind you of all your important appointments.
Synchronize ReminderFox with other calendars as well.

Todo.txt Requires Restart

Thunderbird extension for the Todo.txt application

tasks & mails Requires Restart

Tasks and emails management in a Thunderbird extension. tasks are stored in emails folder and it's possible to link an email to a task.
See all in one, emails and tasks (no more pain to synchronize a task manager).

Bookmarks: eMails and XNotes - Quick 'Jump To'

Bookmark Toolbar and Menu for easy switching to emails and XNotes.

While you can already 'star' or 'tag' emails, they are still hidden in their (deeply nested?) folder and are not in direct daily view.

Bookmark them!

todowebapp

Adds browser action icon to toolbar to open To-Do. See https://thunderbird-essentials.thomas-bandl.de/wordpress/2020/10/04/microsoft-to-do-button/

iCanban - Kanban with ICAL