Weird - solved then problem, whatever it was, by transferring the entire contents of the 'Comics' folder to my Pictures folder. FileZilla had no trouble showing the contents and FTPing to it worked fine.
So I then deleted the now empty 'Comics' folder, rebooted the Xbox, created a new 'Comics' folder (UnleashX File Explorer) and moved all the previous contents back into it from the 'Pictures' folder. Despite being named identically and containing the same folders/files FileZilla had no trouble FTPing to and from it.
Here's the weird thing: prior to all this I downloaded and installed another good free FTP client: CoreFTP LE which I found through a PC magazine recommendation. That worked fine with the original 'Comics' folder FileZilla had the trouble with.
So what was FileZilla's issue with it?