The main new features in this beta release include the addition of touch events for Windows 7 systems with touchscreens, and a JavaScript improvement that should speed up the execution of heavy, numeric code, which is used for things like graphics and animations. Any bugs you encounter while testing Firefox 4 Beta 3 can be reported to Mozilla via the included feedback add-on.
Firefox 4 Beta 3 changelog:
Firefox 4 Beta 3 is available in 34 languages. Web authors can now get touch events from Firefox users on Windows 7 machines. A new way of representing values in JavaScript that allows Firefox to execute heavy, numeric code (used for things like graphics and animations) more efficiently.