For a normal user, the problem is that the <i>Play DRM content</i> checkbox is hidden by default.<p>In order to enable it, you must go to <i>about:config</i> and enable <i>browser.eme.ui.enabled</i>. While you are there, you can just disable <i>media.eme.enabled</i> and <i>media.eme.apiVisible</i>.