XHttpRequest() Gives Erroneous Failure For File://

I'm testing a web site in a local directory by opening index.html from the File/Open File... menu of Firefox. When I use xHttpRequest.send() to retrieve a file to include in my page, the callback's status argument is 8 and req.status is 0. I suppose that the 8 is correct in that the req.status isn't 200 but it seems that for file:// (as opposed to http://) that req.status == 0 might be treated as success. I haven't tested far enough yet to know what req.status would be if the file I tried to retrieve was missing.