The LiveSub add-on for Firefox collects no data and sends nothing to us or to anyone else (its manifest declares "none" for data collection).

What it does with the video's audio: when you start subtitles, the add-on takes the audio of the video playing in a YouTube or Twitch tab and passes it to the LiveSub web page (livesubweb.com) open in the same browser. Speech recognition and translation run inside that page, on your device. Audio never leaves your computer.

What it stores: only the position where you dragged the subtitle box, in the browser's local extension storage.

The LiveSub web page has its own privacy policy, which also covers the add-on: https://livesubweb.com/en/privacy