How do you disable Pocket on Firefox 38 and up with GPO?

For the record I think this is a horrible decision by Mozilla (especially since they had been working on their own alternative called Reading List), compounded by the fact that it's enabled by default for everyone.

Firefox has no native Group Policy support as you might know. There are various free/paid add-ons and solutions to add such support, such as GPO For Firefox, FirefoxADM, Firefox + Group Policy Object, FrontMotion Firefox Community Edition and PolicyPak for Firefox. I cannot personally vouch for any of them however since I've never tried them (IE or Chrome is the way to go in my opinion if ADM/ADMX support is required).

Regardless of which one you decide to go with, what you need to do is set browser.pocket.enabled to false for all your users: