Windows/Mac/Linux: It's not the official beta of Firefox 3.6 — that's been put off for a week. But a "test build" of Firefox 3.6 has landed, with Windows 7 compatible taskbar thumbnails, visual tab previews and a new rendering engine.
You can grab your early build of Firefox 3.6 Beta 1 from Mozilla's FTP servers — select the OS and processor platform from the folder list, and grab the binary inside. There are bound to be some bugs, as you might imagine, and the actual beta is now set to launch October 21, the day before Windows 7 launches.
Along with tab previews in the taskbar, you can check out visual previews of tabs when you switch through them with Ctrl+Tab. It's not enabled by default, but as with the 3.6 alpha, you can enable them by heading to about:config in your address bar, searching for browser.ctrlTab.previews, and setting that value to "true".
Give Firefox 3.6 Beta 1 a try and tell us your impressions in the comments.