I'm familiar with the Same Origin policy, but what I'm wondering is if firefox actually blocks Ajax POST requests from local files. I've been doing some automated javascript testing, and I've enabled CORS on my server, but I actually don't even see the xhr request making it to firebug, which leads me to believe that firefox is completely blocking it.
When I run the same requests in chrome and allow cross domain requests, it works well.
All modern browsers do that : they consider all requests using the file:// protocol as being cross domain.