Pocket is the new name of the "read it later" service. It allows to store web pages in the cloud, and use the same list on all your devices.
It was first available as add-on for Firefox, and got integrated into Firefox with version 38.0.5. It is visible as icon on the toolbar, and as entry in the bookmark menu.
Pocket is not part of the current ESR, because that is based on 38.0.0.
Pocket lately got bad press because security researchers found many serious security vulnerabilities in the service. It was easy to get full access to their servers, and the data of their users appears to be stored unencrypted. They did not replace their certificates, eventhough they must be regarded probably compromised.
Pocket can either be disabled by setting browser.pocket.enabled to false, or by removing the icon from the toolbar. Removing the icon from the toolbar should also automagically remove the pocket entry from the bookmark menu, and the context menu, and as consequence prevent the pocket code from being loaded into Firefox.
However the latter method does sometimes not work because bug 1197871: If you had manually removed the icon from the toolbar, later added it back, and then disable pocket by setting browser.pocket.enabled to false, the entry in the bookmark menu will stay and cannot be removed.
If this happens, you must first re-enable pocket by setting browser.pocket.enabled to true, and restart Firefox. Then make sure that the pocket icon is now really shown in the toolbar. If it is not, you must first add it to the toolbar, by right-clicking in the toolbar, select edit, then drag the button to the toolbar, and quit by pressing the ESC key. Now you can remove it from the toolbar, with a right-click on it, and select "remove icon from toolbar". This will also remove its entry from the bookmark menu.