Hi Mike, the folder contains data stored at the request of websites and add-ons. To clear the website storage, you can use the "Manage Data" button on the Settings page, Privacy & Security panel. It's in the Cookies and Site Data section.

This dialog allows selectively clearing sites whose data and cookies won't be missed, while skipping over those such as AOL where it may be preferable to keep the data. To select sites in bulk, click the first one and Shift+click the last one, then Ctrl+click any exceptions to deselect them (same method as Windows File Explorer).

Thanks - I didn't realize those were the cookies, or associated with them, and thought they might be for the History. My wife said to delete the cookies & the folder shrank to 14MB.

Hi Mike, actually the cookies are stored in a database named cookies.sqlite, and the storage folders contain databases used for "local storage" or "DOM storage", which many sites prefer to use instead of cookies for efficiency reasons. (https://developer.mozilla.org/docs/Web/API/Web_Storage_API)

Thanks for the additional information.

When I go to Manage Data > Clear Cookies that means it clears both cookies & the "local storage", is that correct?

If I block some or all cookies does that also block what goes into "local storage"?

Yes, for many purposes, Firefox groups cookies and local storage (sometimes called Site Data, sometimes called Offline Storage) together because they serve a similar purpose of keeping data for the site on the browser. A site's cookie permission also is used as its local storage permission.