Developing WebExtensions for Thunderbird
You'll approach the coding of an extension for Thunderbird in the same way as you would for a Firefox extension; using a text editor or tool of your choice to write the code.
API differences
Note: See ReadTheDocs for Thunderbird specific WebExtension API documentation.
Being both Gecko based, Thunderbird supports many of the APIs Firefox supports, with some differences, see browser compatibility for manifest.json and browser support for JavaScript APIs for details.