This makes a difference. You won't be able to watch EME-locked content, but every time you try, a content publisher will have to come up with the resources to send you the page with the locked content, all to no avail.
Of course, you'll probably either cancel your Netflix or find a workaround. But you won't be casually supporting EME on sites you never knew were using it. Your every day browsing will automatically block EME.
"The Net interprets censorship as damage and routes around it." - John Gilmore
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.
On most Linux distributions, Firefox is built from source and then shipped via a package manager. At least on Arch, --disable-eme is not passed as a configure flag. Yes, one could build Firefox oneself, but it is a beast to compile. It is also ridiculous that this needs to be done at build time.
A perhaps easier approach that should work based on the name (but have not confirmed) is to lock via the user.js method user_pref/lockPref("media.eme.enabled", false). If this works as intended, then simply make this config file part of the deployment should be enough.
Consumers have the power but not the knowledge.
Users have the knowledge but not the power.
We don't like EME for technical reasons that the average consumer does not understand. Consumers will embrace EME because it connects them to content providers and not doing so would be like the linux users from over a decade ago who couldn't run flash.
We have the power, but sadly not the will.