1) In the address bar of Firefox type:
about:config
2) in the filter type:
browser.cache.disk.parent_directory
3) if this entry is missing,
right click on the blank window, select new-> string
type "browser.cache.disk.parent_directory" as the preference name
4) in the Value dialog box (of the existing or the new entry),
enter c:\temp, providing the foldername in which you wish the file to be located