A couple of changes to example4.html from this great article by Jos Dirksen (http://www.smartjava.org/content/exploring-html5-web-audio-visualizing-sound) are sufficient to get it running in Firefox Aurora.

gistfile1.diff

38,39c38,40
<     // create the audio context (chrome only for now)
<     var context = new webkitAudioContext();
---
>     // create the audio context -- covers Firefox and Chrome
>     var AudioContext = AudioContext || webkitAudioContext;
>     var context = new AudioContext();
70c71,73
<         javascriptNode = context.createJavaScriptNode(2048, 1, 1);
---
>         // createJavaScriptNode now called createScriptProcessor
>         //javascriptNode = context.createJavaScriptNode(2048, 1, 1); 
>         javascriptNode = context.createScriptProcessor(2048, 1, 1);
109c112,113
<         sourceNode.noteOn(0);
---
>         //sourceNode.noteOn(0); // .noteOn renamed to .start
>         sourceNode.start(0);
166d169
<