The answer to Facebook Android and Video Autoplay is, downgrade your Facebook app to an earlier version that doesn’t support Autoplay. Here’s how to do it.
You can’t get old versions of the Facebook app through Play Store so the first step is to disable the security feature that prevents getting apps from anywhere else.
Settings > Security > Unknown Sources
Next, find a source for an older version of the Facebook app. I downloaded and installed three old versions before finding that v3.6.1 doesn’t support Autoplay. There might be a slightly newer version without it but it’s certainly in v184.108.40.206.3. Googling for “Facebook apk 3.6.1” worked for me.
Before doing anything with the above link, uninstall the current Facebook app.
Settings > Apps > Facebook > Uninstall
The reason for not doing this step before is to make sure the older version is still available for download. It would be annoying to delete the new version, only to find there is no old one! Now go back to the v3.6.1 URL you found and download the apk file. It should be about 15MB in size. Once the download completes, click on the apk file and it will install the Facebook app for you.
Now you have an old version, you need to change the auto-update setting to prevent Play Store from upgrading it again.
Settings > Apps > Facebook > untick Show Notifications
Apps > Play Store > Play Store > My Apps > Facebook > Settings (3 dots) > untick Auto-Update.
Lastly, remember to undo the first step by toggling the Unknown Sources option back so that only Play Store will be trusted.
If all has gone well, Facebook will now startup and warn that you’re running an old version and should upgrade. Take delight in ignoring it.