Yes, it can be resource intensive. Use mid models for best performance

SherpaTTS is Foss and works offline after downloading voice data

It’s been here since more than a year. https://blog.mozilla.org/en/mozilla/firefox-androids-new-privacy-feature-total-cookie-protection-stops-companies-from-keeping-tabs-on-your-moves/

It’s there on mobile. Check the details tab in tracking protection. You should see it mentioned there.