Mozilla has released the first beta for Firefox 4.0 (formerly Firefox 3.7). This release is being made available for testing purposes only, and is intended as a preview release for web application developers and the testing community.
Firefox 4 Beta 1 includes dozens of major features and improvements – by testing them early we’ll be able to respond to your feedback for future versions of Firefox. Once you download Firefox 4 Beta 1, you’re part of our beta program and will receive regular updates as more features launch. We’ve included a Feedback Add-On that helps you let us know what you think about the new features and technologies in the beta.
Source: Mozilla Developers News