Cancel an HTTP GET request from Firefox Extension

Question

I have a firefox extension that listens to "http-on-modify-request" and inspects all GET requests coming from firefox. I'd like to be able to cancel the request (say return a fail code to the page) or modify the URI of the request but can't seem to do it. the nsiHttpChannel object just doesn't allow it - for instance

delete httpChannel;

or reseting to an empty request

httpChannel = Components.classes["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance(Components.interfaces.nsIXMLHttpRequest);

don't work (and you can't modify the URI).

So how do you both intercept and modify http GET requests in a firefox extension.

Answer 1

delete httpChannel; just deletes the variable, sort of like saying httpChannel = undefined;. The request itself is unchanged. Similarly, your second idea just sets the variable to point at a new nsIHttpChannel instance, but the old request is still unchanged.

To modify the request, use its properties and methods. See nsIHttpChannel or nsIChannel or nsIRequest. As you say, you can't modify the URI so you might want to cancel it (see below) and replace it with a new one. I'm not sure exactly how to do that but I imagine one of those three pages has the answer.

To cancel it, use nsIRequest.cancel()