There are many ideas for new language features for JavaScript, but it is not always clear whether a new feature will be useful, or what the exact syntax/semantics of that feature should be. Adding these features to SpiderMonkey can be a significant challenge for most developers. Narcissus JavaScript, itself written in JavaScript, provides an excellent alternative, but it has not been available within the browser.

The Zaphod addon integrates Narcissus into Firefox. Developers can write scripts specifically for the Narcissus engine (by specifying script tags with a type of "application/narcissus"), or they can replace SpiderMonkey with Narcissus as the default engine. With Zaphod, language hackers can more easily experiment with new features for JavaScript, and more easily share their ideas with other developers.