It's been one of the areas where Chrome is definitely ahead
Mozilla is finally getting behind the Web Audio API and the latest Firefox Nightly builds support at least some portions of the draft specifications. The API is one of the biggest that Firefox doesn't support at least in some form, that is changing now.Google, Mozilla and, up until recently, Opera are always working to support the very latest web technologies and nascent standards or, alternatively, to impose their own ideas if they believe one particular technology could be useful to developers.
The Web Audio API, pushed by Google is probably in the latter category. In fact, Mozilla worked for a while on an alternative audio API which is still somewhat supported, though deprecated in Firefox.
But Chrome has supported the Web Audio API since Chrome 14, launched in 2011. What's more, the W3C favors it as a future standard.
Since its introduction, there have been several interesting applications built around the Web Audio API, some by Google itself, some by developers working with audio on the web.
Web Audio is designed to make it possible to synthesize music in the browser, or more generically to process audio streams in near real-time with little performance penalties.
The Web Audio API comes in handy for any sort of music app, obviously, but it's also useful for games and any other apps that work with sound and need more than just basic playback.
Firefox Nightly now supports some Web Audio API features, such as the decodeAudioData method. It's going to be a few months before it's ready to be enabled by default, perhaps even more since it's a relatively complex technology.
But it's nice to see progress, support for Web Audio API means Firefox will be able to run games that rely on it, Angry Birds for the web for example, or web apps such as the "virtual" recreations of some of the legendary tools created by electronic sound pioneers the BBC Radiophonic Workshop.