I recently switched from Thunderbird to Postbox. Postbox is built on the same code as Thunderbird.
I love the Todoist Thunderbird extension and would love to see the modifications needed for it to work with Postbox.
Thank you for your feedback, we appreciate it and we'll consider releasing a Postbox plugin in the future. You're welcome to vote for this request in our Votebox: http://todoist.com/Vote/showProposal/462/