It's a free tool and makes rapid prototyping possible. Also, Firefox as a platform is free and widely available. The app itself is fairly good; not as powerful as, say, Balsamiq, but still better than many other options (e.g. Visio).

You can make cross-platform standalone applications with Firefox too: use XULRunner to package your HTML5/CSS3 application. As matter of fact, Firefox itself is built on top of XULRunner. The advantage of XULRunner over AIR is that your users don't need to have AIR runtimes installed.