if you can't see a list of the actual files, there's no way to download all of them (short of just bruteforce trying every combination of characters).
if there really is no list (i.e. no "Index Of" page, etc) then i'm afraid you're out of luck.
if there are multiple lists and theyre scattered and htey link to the files, then you could probably write a script to go to each page/list and consolidate them into one big list which you can then use firefox addon DownThemAll to download.