About this Add-on
You can do the followings:
* Change icon display: Icons beside text, Icons only or Text only
* Change order of toolbar buttons
* Hide unnecessary buttons
* Add spacer and separator
* Add Conversation, Tag and Forward As buttons, which removed from Thunderbird 78
For details, refer to my blog post.
In addition to that some new features are planned. See the following post too.
This is a workaround for the bug 1556261.