An app I use called Zotero uses XUL, though I believe they're trying to transition to Electron. It seemed like a potentially robust platform, but damn if I couldn't figure how it worked enough to even contribute a minor fix.
I think Zotero started out as a Firefox extension, and when Firefox deprecated those in favor of the new-style WebExtensions they saw the writing on the wall and converted it to a standalone XUL application.
The same team behind Zotero has another newer application called tropy (https://tropy.org/), which is based on Electron, and apparently they were happy with that and are indeed planning to transition Zotero to Electron as well.
> I think Zotero started out as a Firefox extension, and when Firefox deprecated those in favor of the new-style WebExtensions they saw the writing on the wall and converted it to a standalone XUL application.
To add a bit more detail / a slight correction: Zotero did indeed start purely as a Firefox extension, but Zotero Standalone was released in 2011[0], far before the switch to WebExtensions. For a while, the Firefox extension and Zotero standalone were equally featured — i.e. you could use just the Firefox extension, or use Zotero standalone + the Chromium connector extension, and in both cases you'd get the same functionality. After the switch to WebExtensions, the Firefox extension was reduced to being purely a connector, just like the Chromium one.
