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IT WORKS WITH THUNDERBIRD 60.* !!!!!!

Just download the AddOn, rename it from .xpi to .zip and open the zip-file
inside the archive is a file called "install.rdf", open it with any text-editor (wordpad, editor, etc.)
look for the line with
<em:maxVersion>53.*</em:maxVersion>
and change it to
<em:maxVersion>61.*</em:maxVersion>
save the file and repack the archive
At the end rename the archive back to the extension .xpi and install it manually with thunderbird

Это отзыв для предыдущей версии этого дополнения (11.0.4.1). 

You can do that, but it doesn't mean that the add-on will work totally fine. The first thing I did when TB60 was released is to upgrade the compatibility number in the install.rdf file to check my add-on. This time I found it is too buggy, some icons are missing and the code came to the point that it is simply too bloated.

In addition, I would like to make use of CSS variables to make it a tad easier to maintain/edit my add-on and I also would like the theme's icons to support HDPI displays, since how this kind of screens are more and more popular. And finally, I would like to add official support for MacOS platform.

That's for all the reasons mentioned above that my addon's code needs a substantial refresh.

So edit the install.rdf file if you want, but keep in mind what I wrote above.