In the case of node-webkit they give you access to both the Web platform and node APIs. That's possible because both projects use V8, so with a bit of twisting

In the case of node-webkit they give you access to both the Web platform and node APIs. That's possible because both projects use V8, so with a bit of twisting they can be relatively easily integrated into one. (Incidentally, this is why they're using io.js, the outdated V8 version in node made things more difficult) If you wanted node-gecko you'd either need to fuse V8 to gecko or reimplement or create a wrapper around spidermonkey emulating the V8 API. That's a lot of work.