What this means is that tabs within Firefox will show up as previews when you hover over Firefox' taskbar entry in Windows 7, much in the same way Internet Explorer 8 already does it. This makes Firefox one of the first high-profile applications (that I know of) to offer integration with Windows 7's new taskbar.
Firefox 3.6 also comes with visual previews when you use the
ctrl+tab method to switch tabs - all in a very Windows 7ish coating. This feature is disabled by default, so you need to dive into
about:config, find the
browser.ctrlTab.previews key and set it to
true. Restart Firefox, and experience the new tabbing feature in all its glory.
The new tab switching feature has an additional nicety which probably comes from Andy Razkin: apart from showing visual previews of the tabs, the new tab feature also shows an option called "Show all n tabs". This will bring up a permanent preview window with an instant search field to search through your open tabs.
Firefox 3.6 also comes with performance improvements, but for the most part, it's an evolutionary release. On the Mac OS X side, the roadmap for 3.6 includes native dictionary integration, services and AppleScript support, and Keychain integration. The roadmap has nothing to say about Linux-specific features. Get it from Mozilla's FTP server.