As you are probably aware, Mozilla has recently adopted a new accelerated development cycle for Firefox. With that, they have created a new Aurora channel and Firefox 5 has been in the Aurora channel for a while. Yesterday, Mozilla has finally decided to bring Firefox 5 to the beta channel.
Firefox 5 Beta also includes a channel changer. The Channel changer can be used to change between the Aurora Channel, the Beta Channel and the Release Channel. The release Channel contains only the stable releases - that is Firefox 4 for now.
Firefox 5 Beta also has a new memory monitoring interface which can be accessed by entering about:memory in the awesome bar.
In Firefox 5 Beta, the Do-Not-Track has also been moved to increase discoverability. This, of course, still relies on the participation of the advertising networks.
Linux users will also be happy to note that Firefox 5 Beta brings better desktop integration.
You can download Firefox 5 Beta from here.