Firefox downloads previously downloaded versions of zipped files rather than current version.
On a website I manage, I downloaded a zipped MS Word file. After making changes to the file and uploading the modified file to my server, Firefox still downloads the old version of the zipped file. In fact, if I rename the file (so the link now points to a file that no longer exists on the server), the previously downloaded version of the file is STILL sent. I've used two methods (FireFTP and the server company's file manager) to rename the file, and the non-existent file is still sent.
Re: Firefox downloads previously downloaded versions of zipped files rather than current version.
A reload and bypassing the cache makes Firefox send specific no-cache headers that may help in case data is cached elsewhere. It is possible that the server on the website is still sending old files if you really get this file and clearing caches in Firefox doesn't help.