This is accomplished through HTML5
"With this new API," reads a post on the Mozilla blog, "developers can read and write raw audio data within the browser, presenting audio information in completely new ways that could allow, for example, for people to visually experience a speech or a song through Firefox."
This means that you can experience audio in Firefox in a variety of new ways -- as a rapidly scrolling string of raw-data numbers, as an audio wave visualization, as a filtered stream of sound, as a sound spectrum on an EQ interface, as an abstract 3D animation and much more.
And if you're a Windows user, you can expect better, faster graphics performance as well. The broswer "takes advantage of the built-in graphics hardware in Windows computers with DirectX 10 to improve graphics performance," according to Mozilla. "On supported hardware, Firefox will use Direct2D by default to speed up the display of content on graphically intensive websites."
The new version of the Firefox 4 Beta is available for download today, free of charge.
David Humphrey is one of the primary developers responsible for Firefox 4 Beta's new audio features. Here's a quick demo he made to show how the new audio features and capabilities work: