Except stable version, Firefox also release beta version to be used by public. Usually this beta version will be the next Firefox version. For example now, Firefox release Firefox 3.6 beta and Firefox has 3.5 stable. In future Firefox 3.6 will replace Firefox 3.5. You can try this beta version, find any bug and report to their developer to be fixed on next version.
But you can’t find beta version on Firefox homepage. So where can I download Firefox Beta version?
You can do that by visiting Firefox Beta Version
Try the software and take a risk by yourself. Report any bug to help them make Firefox better.