Speech to text in the cloud is a hard no from me. Especially if it’s Google’s speech to text.
On a serious note, it does bother me how much Mozilla constantly uses Google, even when they have their own solutions They could easily choose not to, especially with their massive budget, but often don't. They have their own Voice API, but they use Google's. They have their own location API, but they use Google's ('use my location' sends your info to Google in Firefox). They have thrown Google analytics into browser components before, and used it on their own websites.