Firefox plugin that intercepts certain HTTP requests

Is there a way to make a Firefox plugin intercept certain requests and respond to them without hitting the network?

Example: every time the browser is about to request http://www.example.com/page.html, the plugin automatically returns "Hello world!" for that page.

Cheers!

How do I block a Firefox request?

Blocking a specific URL from the request list. Hover over the item you want to block in the Request List. Select Block URL from the context menu.

Tamper Data is a good one.

Use tamperdata to view and modify HTTP/HTTPS headers and post parameters.

Security test web applications by modifying POST parameters.

You can checkout tamper data sources and see how the extension works in mozdev and borrow some ideas, or get in touch with the author in adamsplugins [AT] gmail {DOT} com.

I don't know if it's exactly what you're looking for, but it's really powerful. I use it a lot with Firebug, so you can edit your pages on the fly and test for some behaviors.