Originally, there was XULRunner, which was used in applications including ChatZilla, Google AdWords Editor, Komodo Edit, Pencil, Songbird, and Zotero. XULRunner

Originally, there was XULRunner, which was used in applications including ChatZilla, Google AdWords Editor, Komodo Edit, Pencil, Songbird, and Zotero. XULRunner has been discontinued. https://developer.mozilla.org/en-US/docs/Archive/Mozilla/XULRunner 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 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 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 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/Embedding_Mozilla/FAQ/Embedding_Gecko 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.