Hello,
I’m trying to develop extension for Thunderbird and not Firefox. The Firefox part is well documented, but not the Thunderbird one (how to interact with the different menus, windows and toolbars, which seem different from Firefox).
Have I missed something?
Thanks a lot 
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It was decided in bug 1503501 that Thunderbird WebExtension API documentation would be kept outside of MDN.
Thank you for this answer.
It’s a bit hard to encounter and discuss with add-on developers