As new Thunderbird user I was exploring its UI and noticed (besides rather basic HTML typically used in rich-formatted emails) that add-on manager opens a tab with https://addons.thunderbird.net/ and allows a limited browsing (no back button and address bar) of that website. The rendering of the website sort of implies that I view it thru fully-featured (or limited) web browser. Given Thunderbird is a sister project to Firefox and having a similar architecture, I can come to conclusion what this Thunderbird component is Mozilla Gecko. Could someone please positively (or negatively) confirm that? And maybe add some explanation on the "internal browser's" capabilities and limitations. Unfortunately, Thunderbird is very poorly documented, even developer's article on that subject has not been written yet.
So far this question is purely theoretical, but if it is really fully functional Gecko there could be some uses for it, eg. viewing PDF attachments via built-in
pdf.js or even submitting basic subscribe/unsubscribe web-forms. But this is rather broad topic, let it be the separate discussion.