We're working hard on the spec and its test suite to make it easy for other browser vendors to support the API, so hopefully we'll see support for it in more browsers soon.
Disclaimer: I co-chair the W3C Web Audio WG.
I've had this rant before but WebKit is becoming the new IE6 and it's pissing me off as a Firefox and IE user.
Sorry you're pissed off, but your rant is directed at the wrong people. Go file bug reports with Mozilla and Microsoft instead of chastising someone for making something cool.
I haven't checked that this is the case (i.e. whether making it work in Fx would be huge burden or even impossible), possibly not. Anyway, just saying.
EDIT: And to be precise, XMLHttpRequest is Mozilla's naming that came later (after Microsoft's interface name: IXMLHTTPRequest). So you meant: ActiveXObject("Microsoft.XMLHTTP")