Opt out of CDM playback, uninstall CDMs and stop all CDM downloads
You have the choice to globally opt out of HTML5 DRM playback. Once you opt out, Firefox will delete any downloaded CDMs from your hard drive, cease all future CDM downloads and disable DRM playback. This affects only DRM-controlled HTML5 audio and video. To opt out of HTML5 DRM playback completely, follow these steps:
Click the menu button and choose Options.
Click the Content panel.
Remove the check mark next to Play DRM content.
For a normal user, the problem is that the Play DRM content checkbox is hidden by default.
In order to enable it, you must go to about:config and enable browser.eme.ui.enabled. While you are there, you can just disable media.eme.enabled and media.eme.apiVisible.
Thanks for the correction. I forgot that EME support (using Google's Widevine CDM) won't ship on OS X until Firefox 47, which is still in the Firefox Beta channel for another few weeks.
Opt out of CDM playback, uninstall CDMs and stop all CDM downloads
You have the choice to globally opt out of HTML5 DRM playback. Once you opt out, Firefox will delete any downloaded CDMs from your hard drive, cease all future CDM downloads and disable DRM playback. This affects only DRM-controlled HTML5 audio and video. To opt out of HTML5 DRM playback completely, follow these steps: