There was Songbird [1] and Postbox [2] at least at some point was based on Thunderbird. I believe, Zotero [3] still uses XULRunner. I'm sure there were more.
> Many of these either vanished or transitioned to something else (I think both Komodo and Celtx did).
I know Celtx decided to drop the concept of desktop app altogether and went full web-based-only SaaS.
The desktop Celtx app was a good screenwriting program that was easily affordable for hobbyists and the pivot to a paid subscription service with less focus on just screenwriting and more focus on other preproduction activities left a small hole for a short while. (These days the Fountain ecosystem seems the best suggestion for the hobbyest screenwriter.)
The Palemoon browser [0] also still uses XUL, and is in many ways a continuation of XUL browsers (was originally forked from FF 29, updated with various components from FF 50+, and with many other tweaks).
[1]: https://en.wikipedia.org/wiki/Songbird_(software)
[2]: https://www.postbox-inc.com/
[3]: https://www.zotero.org/