Right now you can only help the STT engine by contributing to commonvoice.mozilla.org, where the samples are published to the world. The add-on will now, if you

Right now you can only help the STT engine by contributing to commonvoice.mozilla.org, where the samples are published to the world. The add-on will now, if you opt into it, keep the data in only Mozilla's and Google's hands. Mozilla has an agreement that Google won't keep the data, but even if Google doesn't comply by the agreement, the number of parties with access to the data will be lower than "everyone".