Mozilla has released the fifth beta for Firefox 4.0 on September 7th (along with updates to Thunderbird and other versions of Firefox). 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.

New stuff in this release:

• Support for the new proposed Audio Data API
• Direct2D Hardware Acceleration is now on by default for Windows 7 users
• Firefox button has a new look for Windows Vista and Windows 7 users
• Support for HSTS security protocol allowing sites to insist that they only be loaded over SSL.

For more info see the release notes for Firefox 4 Beta 5. At this time the next planned release for Firefox 4.0 is RC1 in the later half of October. However, given there is still 6-8 weeks until this release, there is a chance there could be a Beta 6.