The Same origin policy ensures that your browser won't load files from sources other than the original source of the loading file. It's vital to help prevent cross-site scripting attacks, but a bit annoying when you're working to draft AJAX responses.
One way around this is to run a web server on your local machine. There's lots of circumstances where this isn't ideal though, such as on an underpowered Windows piece of crap like I use at work.
Fortunately, Firefox has an option that helps get around this. I didn't find it when searching for "Disable same origin policy", so hopefully this blog post will help future seekers.
The Firefox option security.fileuri.strict_origin_policy allows local files to bypass the same origin policy. Yay!