Rated 2 out of 5 stars

This is an excellent extension to use with Firefox as new buttons can be added to the toolbar Customise option simply by selecting a link on the home page and installing them. However, the documentation is obscure for installing extra buttons for non-Firefox Mozilla applications such as Thunderbird, and installs from the home page automatically add the buttons to Firefox, not other applications. That is why I'm giving this a low rating for Thunderbird - it is not obvious just how to add pre-existing buttons to the application!

This review is for a previous version of the add-on (3.0.1). 

Laura, glad you like the extension. Sorry you had issues installing into Thunderbird. I believe we answered this question for you when you posted on the forum.
http://custombuttons2.com/en-us/forum/announcements/extension-announcements/faqs-extension-faq-comments-suggestions-and-issues.html#comment-825
You are welcome to update the rating if you are so obliged. We will be adding that overlooked piece of documentation as time permits.