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.
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.