The latest pre-release version brings several new features, including a new tab organization tool dubbed "Panorama" (previously known as Tab Candy), which allows you to group and quickly switch between different sets of open tabs that share a similar purpose. The feature can be accessed through an icon on your Firefox toolbar or with the keyboard shortcut Ctrl-Space, and will come in handy for those of us who frequently have dozens of tabs open at the same time.
Additionally, Firefox 4 beta 4 brings support for the HTML5 video buffered property and hardware acceleration in Windows 7. The latter won't be enabled by default, though flipping the switch should be pretty straightforward (instructions are available here). The Mozilla team aims to release a fifth beta this Friday, and a feature-frozen beta 6 on September 10, before moving on to the final code sometime in October or November.