Are you using the Clear Data button and clearing both Cached Web Content and Cookies and Site Data? Does the Manage Data button show any data storage in use at that point? If it's not site data, it could be Firefox data. Firefox frequently retrieves data in the background, so that could be what appears in the cache as soon as you clear it. To see the contents of the disk cache in more detail, type or paste '''about:cache''' in the address bar and press enter to load that internal summary page. From there you can click into the disk cache index.