Get Policeman addon, go to Manage rule sets: Enable "Allow any request" Then go to Edit custom rules, and add a persistent rule: Reject media, leaving origin and destination fields empty.
You can't now watch any video on any website, they won't even get downloaded, regardless of Javascript settings, Video tag and else. For even more data savings just let Policeman act in default deny mode, and whitelist things you need. It's boring to create the whitelist but you can spare a lot of data traffic, and it's only done once.
