Accepting online does not imply accepting closed-source. So IMHO that’s a no-go area for anything Mozilla ships as “part of the product’s functionality”, unless we offer the user multiple options for their speech engine, just like we offer multiple options for their search engine. Hosted speech engines may not currently be competing, swappable services in the same way search engines are, but maybe they will become more like that in the coming years.
A client only for Google Speech API is not what I would call a user agent; even Google Chrome allows setting DuckDuckGo as your search engine.
If we give the user the option to send all live sound that follows the wakeup-word to a closed-source, single-instance, hosted, untrusted speech engine, then we should at least give them 2 or 3 options to choose that speech engine from, like Firefox does with search engines.