Firefox caches files from a web page, presumably saved somewhere in the .mozilla/firefox/[profile]/Cache/ directory under a random name. It is possible to view information about the cached files and save a copy using a more descriptive name.
Go to about:cache?device=disk in the browsers location bar. This will display a page called "Information about the Cache Service" which contains a list of links to the files "Cache entry information" pages. These pages will tell you where each file is cached on your hard disk - look for the "file on disk" entry. Rename the cached file to be the same as the original.