If you’re getting annoyed with videos posted on Facebook playing automatically like me, here’s how you can stop this once and for all –
1. Go to your Facebook Settings
2. Go to Videos options found at the bottom of the list of options
3. Change the Auto-play Videos setting to Off.
..and you’re good.
For Facebook app’s auto-play, you need to access your phone app’s settings. You can follow this guide: How do I stop videos from playing automatically in News Feed on my mobile phone?