Eh. XUL isn't really a web technology the same way as Electron.

It's not XUL anymore, actually, as far as I know. That's why it ripped out support for XUL addons.

Firefox has been ripping out xul, but Thunderbird still appears to support it.

Only if you're using a ridiculously outdated copy:

changed

Add-on support: Add-ons are only supported if add-on authors have adapted them

changed

Dictionary support: Only WebExtension dictionaries are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension dictionaries.

changed

Theme support: Only WebExtension themes are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension themes.

https://www.thunderbird.net/en-US/thunderbird/68.0/releaseno...

Literally here's a doc explaining how XUL has changed as of Thunderbird 68, the most recent version, released about a month and a half ago. Yes, some elements have been removed, but others have been modified and still exist.

https://developer.thunderbird.net/add-ons/updating/tb68/chan...

And that's in the add-on documentation, not even just internal development docs.

Also, describing information changed in the most recent stable release, a month and a half ago, hardly qualifies any older as "ridiculously outdated ".

Last time I looked, Thunderbird was about:

    - 1/3 C/C++
    - 1/3 Javascript
    - 1/3 everything else (XML, CSS, etc) known to humanity

I'll grant you that an Electron app is generally 90% C++ (ships a web browser), but I'm not sure if that makes Thunderbird (ships a web browser) any better.