The new version includes Personas, which lets people customize the browser's appearance; blocks third-party software from encroaching on its file system turf to increase stability; and perhaps most significantly given the competitive threat from Google Chrome, shortens start-up time and improves responsiveness and JavaScript performance.
Firefox 3.6 RC1 is available from Mozilla's download site.
People can notice skins and better performance, but there also are changes deeper under the hood that developers should know about. One is support for the File interface, which can help with tasks like uploading multiple photos and is part of the draft HTML5 standard effort. Another deeper change is running scripts asynchronously, which can help load a Web page faster by putting off some work until the high-priority chores are complete.
Mozilla had hoped to release the updated browser in 2009 as part of a higher-frequency release schedule, but gave itself a bit more time for Firefox 3.6 and 4.0.