Resolving a custom-made URL to a custom-made page (without redirecting, without changing the URL name)

Hey,

I’d like to make an extension that takes the (fictional!) URL 'http://joke.hahahaha/', and shows a simple HTML page with the body “this is a joke ha ha ha ha” (for example).

I could in theory use webRequest.onBeforeRequest, and then redirect to a webpage that shows the desired html. But then the URL is not 'http://joke.hahahaha/', and it kind of loses the point.

Is there any way to do what I want?

The extension is for personal usage (for now), so solutions that include changes in about:config or stuff like that are ok.