Is it possible to write an add-on that grabs the audio stream as it emerges from Firefox and manipulate it before sending it on its way?
If so I will pay a programmer hacker that knows how to do that, and simply prove he can do that, to create an add-on that merely connects to a single VST host and lets it play havoc any way a creative sound engineer can provide before passing it on to whatever bit bucket Firefox sound usually goes into.
If you are interested you can trade payment for equity. For 100% equity, no payment, it’s all yours, child, I just get the joy of helping make it and using it to solve another problem I’m brickwalled against. I think there is money potential enough (it will become a ubiquitous if not native capability) by itself that somebody who know how should jump on it. If it fails to spark that inevitability feeling in anyone I will shell out some of my hard saved to get it. I can be a useful mentor as well to help you learn the lay of the land.
Even if there is not a current spot in the add-on API for processing sound I can’t imagine the hack to provide it would prove to even be a difficult task for someone with the right skill set. I’m willing to even consider a FF executable patch with legs. Then one needs to be able to, from the add-on’s JavaScript, invoke an external interface to exchange an audio stream to that, a VST host with a GUI running on the user’s machine. There are many superlative candidates for that host function. The add-on needs provide nothing for management. It’s only user interface is the ability to name a VST host that’s on one’s machine. Then wait for the contributions to roll in. 
Even if you just carry the task to where any JavaScript programmer can use your new API as a filter I might then be able to find someone who can do the rest regarding hooking up with a running app to hand it samples and return the results to the Firefox stream.