Steps: 1. open about:preferences#privacy 2. scroll down to "Site Data" 3. click "learn more" AR: Link goes to "Page Not Found" page.
See bug 1313602 comment 10 from Joni Savage. An article titled "Storage API - some websites want to add files to my storage" was created for SUMO on Lithium and can currently be found here: https://hwsfp35778.lithium.com/t5/Personalize-Firefox/Storage-API-some-websites-want-to-add-files-to-my-storage/ta-p/1380537 It doesn't look as though this article was recreated in the rollback to Kitsune. I'll NeedInfo Joni.
I NeedInfo'ed Joni in bug 1313602 so I'll remove the request from this bug.
I'm marking this bug fixed per bug 1313602 comment 14. Joni recreated this article on Kitsune and had the inproduct URL redirect to the new link. You should see this article when you click on the inproduct link now: https://support.mozilla.org/kb/permission-store-data