I thought that Thunderbird was a native application for quite a long time. For some reason it never had the same icky feeling I get from any Electron application (even supposedly "high quality" ones) and worked fast on crappy machines (which is what I've typically used throughout the life). Of course, performance went a bit downhill since they abandoned XUL in favor of pure HTML, but still.

Reading the article, I was expecting the dreaded “and we’re rebuilding it on Electron” (because apparently that’s the only way anyone knows to build a desktop app nowadays), but apparently they don’t plan to move to an entirely different technical base, just to rewrite stuff.

No, because they have already done that (except on Firefox instead of Chromium).

Do they use JavaScript outside the UI?

The SMTP client is now written in javascript and most of the IMAP client as well.

The whole calendaring system is written in javascript.

The chat clients (irc, matrix, etc) are also written in javascript.

Okay, so not that dissimilar to an Electron stack after all.