Is there a similar component for the Firefox JS engine? I'd be interested to know if it could be used standalone and how big it would be. (Mozilla also has the Rhino JS engine that works on Android, but this is something different)

https://developer.mozilla.org/en-US/docs/Archive/Mozilla/XUL...

Mozilla briefly developed Positron, which was an Electron-compatible app shell that used Gecko instead of Chromium/Node.js/V8. Positron has also been discontinued.

https://github.com/mozilla/positron

The developer who was working on Positron went on to develop qbrt, which is a CLI interface to a Gecko app runtime. The project is unstable, and the last commit date is Jun 30, 2018.

https://github.com/mozilla/qbrt

Another Mozilla developer has a project called servoshell, which is an embeddable version of Servo. It's described as work-in-progress, and the last commit date is Jan 7, 2018.

https://github.com/paulrouget/servoshell

I'm afraid this is it for now, since embedding Gecko is no longer unsupported by Mozilla.

https://developer.mozilla.org/en-US/docs/Mozilla/Gecko/Embed...

Hopefully, Mozilla will continue working on embeddable versions of desktop Gecko after they release the necessary improvements in Firefox.