How to create add-on for Mozilla Thunderbird desktop app?

I am beginner to Mozilla Addons Development. I want to create add-on Mozilla Thunderbird desktop app. I researched how to create it but i found only browser extension.

Where should I start?

Thanks. I would be very thankful to you.

Thunderbird add-ons are now hosted on https://addons.thunderbird.net/en-US/thunderbird/.

Have a look at https://developer.thunderbird.net/add-ons/upgrading-add-ons-for-tb68 and post followup questions to https://discourse.mozilla.org/c/thunderbird/addons