Narcissus came about as a way of demonstrating rules for new functions by programming language researchers instead of being forced to use a formal grammar notation. Instead this is implemented in Narcissus so it's possible to see how the code will behave without having to deal with a massive code-base. Zaphod then becomes useful for distributing and testing your Narcissus targeted code in a browser.
<meta http-equiv=”Content-Script-Type” content=”application/narcissus” />
One thing to note is that at present if you use Zaphod to disable SpiderMonkey as well it will mean that sites such as Gmail will no longer function. This is because Gmail uses functions which are not readily available in Narcissus.