I believe you missed the part where the codebase has become impossible to work on. This isn't change "for change's sake", it's "because if we don't, this product is dead".
Thunderbird is absolutely due for a UX and feature set overhaul. But please please please don't follow current design trends: Thunderbird users are not and will probably never be people who appreciate the "everything is a mobile app" fad of the last few years. The term "webshit" absolutely feels apt here, and I would gladly triple my donation for the coming year if it meant that they committed to maintaining a traditional desktop look and feel in the rewrite.
I will probably need to look for alternatives, if I catch any sniff of "we are an electron app now" or similar. In that case I will probably only use the older version as a lookup program for past e-mails, if I cannot import them into something else. Or I stick with an old version of Thunderbird.
XUL is still hiding in there I think, though most things in Firefox have been rewritten in regular HTML. Not sure about Thunderbird, though the new settings page suspiciously looks like the Firefox one, so if not everything has been migrated yet, some stuff definitely have.
Either way, Gecko is what renders XUL and HTML, and therefore Thunderbird. It's essentially a big chunk of web stuff. It has always been this way.
Though I think Gecko is a tad more memory efficient than Chromium / Electron, and a lot nicer to use than your average Electron app.
And I'm pretty certain Thunderbird is staying this way. Becoming an Electron app would be a huge rewrite, and I think Thunderbird devs like Gecko. They say it in the blog post. It's not happening.
