There is a stable version of Firefox (version 3.6), you can download it at http://www.mozilla.org/firefox/. Mozilla is also adding new features to Firefox in an effort to enable web developers to do new things in new ways. Development and stabilization of these new features is ongoing, which is why it's restricted to a beta program in the next version.
What would you prefer? That Mozilla abandon innovation completely and concentrate utterly on stability? That you not be allowed to participate in beta testing the next version of the browser? If you don't want to test Firefox's new features in the unstable version then perhaps you can avoid running the beta and just stick with the older version.