It's the same model used for the WebSpeech API, which can operate entirely offline.

Google mostly funded the training of this model around 10 years ago, and it's quite good.

There are many websites that are simple frontends for this model which is built into Webkit and Blink based browsers. However to my knowledge the model is a blob packed into the apps which is not open source, hence the no Firefox support.

https://developer.mozilla.org/en-US/docs/Web/API/Web_Speech_...

https://www.google.com/intl/en/chrome/demos/speech.html