FireFox 3.5 was just released about a half hour ago. You can checkout all the new features for web developers here.

For me, as someone that does a lot of security research one of the most interesting new features is the Origin http header that FireFox 3.5 now sends. The Origin header when your browser makes a request the following types of requests: scripts, stylesheets, form GET & form POST, redirects, XMLHttpRequest (XHR, ajax), and frames.

You may be thinking, ok how is this different than the HTTP Referrer header. First, it only sends the domain name of the page, and second it doesn't have many privacy concerns (so hopefully people won't turn it off).

So how can this improve security?

Web Servers can block requests that send invalid Origin headers, this will mitigate the risk of cross site request forgeries (CSRF), including JSON hijacking for people using browsers that support this feature.

You can read more about the origin header here.