If you selected, as you downloaded the file, a custom place to place the download, Firefox will have stored the file there (if the download was successfully completed). If you forgot where it was, you can go into Tools > Downloads, rightclick the file you want, and select "Open Containing Folder". If Firefox did not ask you where to save the file, you go into Tools > Options > General and select the location where Firefox will store your downloads. Here, you can also configure it to ask you every time you choose to save a download. As before, you can go into Tools > Downloads and ask for the containing folder. If you simply opened the download without asking Firefox to save it, it's possible that you can still access your file (if you did not close Firefox in the meantime). Files are are opened and not stored, are temporarily placed in a cache folder so you can use it while your Firefox session lasts. Simply go into Tools > Downloads, and, as before, ask for the containing folder. From there, you can copy/cut the file and paste it anywhere you want.