Unfortunately Firefox 3.6 on the Mac shipped with a bug that prevents Freeway users from previewing their pages using the browser. It appears the issue is related to file paths that contain spaces so, for example, if the path to the page you are designing in Freeway has a space in it then Firefox will silently ignore the request.
To allow Freeway users to keep using the browser from with the application I've created a very simple AppleScript droplet that patches the file path and opens the requested file for you. Simple!
Simply download the droplet, and in Freeway set it as a preview application (File/ Preview in Browser/ Browser Setup...). Now when you want to preview in Firefox simply select the Firefox 3.6 Preview droplet and your page should appear in Firefox as expected.
(Apologies to Jon Hicks for crimes against his great Firefox icon).