You would need to ask for confirmation and specify where and under which name to save the file.

• Tools > Options > General : Downloads : "Always ask me where to save every file"

• https://support.mozilla.org/kb/Downloads+window

alternatively the DownThemAll download manager addon would allow to set that in its options panel: https://addons.mozilla.org/firefox/addon/downthemall

Just be careful with automatically overwriting files. Sooner or later you will inadvertently overwrite a file that that is completely different, but happens to have the same name.

This answer is not solve my problem. If I downloading loads of books in one folder , and if I click to download a book that I downloaded a few minutes before I would like to firefox warn me that I already downloaded this book instead of downloading again the same book and putting on the end (1) or if I download it again (2). I would like to prevent me from a double download. In the older version of firefox this was automatically done, and I wandering how can I do it now in the new version.

That has never been a standard feature in Firefox, as far back as I can recall. You probably had an add-on which added that feature to Firefox.

Have you tried DownThenAll?
Or tried what cor-el suggested with using the "Always ask me ..." pref?

philipp and the-edmeister this really helped . DownThemAll! is the right answer for my problem. Thanks for both of you