This version is based on Gecko 1.9.2 web rendering engine which contains many improvements for web developers, Add-on developers, and users. It now supports new CSS, DOM, HTML5 and the WOOF font format.
Mozilla also added a new technology that alerts users about out-of-date plugins as well as a tech that enables users to drag and drop files from the PC to the browser, making it easier for developers to update websites.
Most add-ons however are yet to be upgraded by their authors to be compatible with Firefox 3.6 beta. Users who want to test their add-ons can download the Add-on Compatibility Reporter here.