Mozilla has released the fourth beta for Firefox 4.0 on August 24th. 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: Firefox Sync is now included by default. A new feature gives users a visual overview of all open tabs, allowing them to be sorted and grouped. An experimental API is included to provide more efficient Javascript animations. Firefox now supports the HTML5 video “buffered” property. See the complete changelist from the previous beta. For more info see the…