please confirm if you're following these exact steps:
1. Click on "Show projects" in the Todoist plugin window.
2. Choose a project to enter it.
3. Open an email in Thunderbird.
4. Scroll to the bottom of the task list.
At this point, the "Add email as task" link should be visible next to the "Add task" link.
If it's not, please tell me - which version of Firefox is installed on your computer?