How to upgrade a Thunderbird 20-58 Addon that uses XUL?

I would like to update my addon
that uses XUL.

Where do I start?

Firstly, there were some changes required for TB 60. Take a look here:

Then there were more significant changes required for TB68:

And the best forum for addon development is now here:

Markus, you should also read this:

That means, if you want that your addon will keep working after the next major update, you should convert it to a “MailExtension” or a “Web Extension Experiment”.
Most probably the API’s needed for a “MailExtension” are not yet available, so most probably the way to go is to convert your addon to a “Web Extension Experiment”.

If you like, you can use my example “Search Button Addon” as a starting point and put in your code: