If you were waiting for the version of Firefox that takes advantage of your new ATI or Nvidia GPU. If you've got Windows 7, you'll want to check out the latest Firefox 4 beta.
While the previous release supported Direct2D features in a secret setting, this latest beta release, available for download here, has it enabled by default.
This new Firefox 4 beta also introduces a new audio API to expose the raw audio data housed within the <video> and <audio> elements in HTML5 so that developers can read and write raw audio data within the browser.
Read more about the latest beta here.