Although i have never had problems with the RC's i suggest
you only try this if you like bleeding edge software.
Firefox 1.5 Release Candidate 1 is now available for download. This is the first release candidate of our next generation Firefox browser, to be released later this year, and it is being made available to our developer and testing community for compatibility testing and to solicit feedback.
Note: This is not the final release of our Web browser, it has been made available for testing purposes only, with no end-user support. If that sounds scary, you'd probably be better off with the latest version