The add-on could tell Mozilla which files to keep because it saved the decryption key for those files. Mozilla could then select those files to decrypt, e.g. to prove to authorities that its file-sharing service was not used for illicit purposes. Alternatively, the add-on could filter the IP addresses used to upload the files for potentially sensitive blocks and then tell Mozilla to decrypt the files uploaded by people from such blocks, e.g. in an attempt to engage in corporate espionage (of course one shouldn’t use third-party services for sensitive files in the first place, but if you have to use a third-party service, certainly an ‘open-source’, ‘private’ and ‘encrypted’ one from such a reputable company as Mozilla, right?)