The main profile location in "AppData\Roaming" is for persistent files. Temporary data like the disk cache and other caches including safe browsing data that is used internally is stored in "AppData\Local". The storage folder is used by IndexedDB, this data includes storage used by extensions and you would lose this data if you use the refresh feature in Firefox that only preserves the main location in the "Old Firefox Data" folder on the desktop. You can cleanup the storage folder if it grows too large or if you notice websites that you haven't visited for quite some time and that you do not intend to visit.

Thanks for the reply, but I don't have any extensions, I don't care about having data from any sites available offline, and it looks like pretty much every website I've visited stores "something" in the C:\Users\USERNAME\AppData\Roaming\Mozilla\Firefox\Profiles\PROFILENAME\storage location.

End result, on my own computer that location has 123MB of data consisting of 5016 files and 3919 folders. The 123MB of data isn't much of a problem, but the huge quantity of files and folders really slows down the roaming profile sync to the server. Every other Firefox user I've encountered has a profile with at least that much, if not 2x or 3x as many files and folders in the storage location.

Why isn't this data stored in the "Local" portion of the Appdata folder in the user profile instead of the Roaming" portion so that it doesn't get synced back and forth with roaming profile usage? From everything I can tell, not one single person has missed any of the data when I empty out their "storage" folder.