See http://kb.mozillazine.org/Password_Manager (Troubleshooting)

Firefox 3.5/6 use signons.sqlite to store the names and passwords. The matching file key3.db that stores the encryption key is also required to access the encrypted names and passwords.

Thanks for your answer! I will try to write my comment in english. I found the file key3.db but I don't try to read it (with which software?). In fact, all my passwords have been saved on a paper document. I just would like to know how these passwords have disappeared from Firefox and how to avoid these bug. Did I made a wrong manipulation? Thank you for your assistance in this matter

If Firefox no longer asks to save a name and password if you visit a website and log in then there may be a problem with the files signons.sqlite and key3.db. As a test you can move or rename those two files to see if that makes it work again.

I found a file key3.db here : c:\Program Files\Wanadoo\Config\xxxxxx\Firefox. But as I am no longer with Wanadoo but with another ISP, I don't understand why this file is there. And I don't find the file signons.sqlite on my hard disk. May I suggest to uninstall Firefox and reinstall it? In this case, what about my favorites?