Gecko, which is the rendering engine used by Firefox, has existed for longer than Blink/Webkit (powering Electron). It also supported a UI toolkit (described in

Gecko, which is the rendering engine used by Firefox, has existed for longer than Blink/Webkit (powering Electron). It also supported a UI toolkit (described in XML and a wrapper on Gtk etc.) called XUL [1]. I think OP is talking about this. Mozilla never tried to make it easy to use Gecko (plus the Spidermonkey JS engine) as a re-usable library outside Firefox, making something like Electron really difficult to do. On the other hand, Blink is a lot easier to integrate, which is why Electron exists (as a thin wrapper over Blink+V8). [1]: https://en.wikipedia.org/wiki/XUL https://en.wikipedia.org/wiki/XUL