Right now, the only item that can be attached to a Task is a webpage, but it would be helpful to be able to drag an email / email thread from the Thunderbird's main window into the Attachments panel when editing a task.
This feature would help provide context for the task (when coming back to it later) and also make it quicker to find the email thread that requires a follow up.
This feature could even replace / rework the Convert To > Task feature for an email, which simply copies the email body to the task description.