Looks like you have forgotten your Master password that is used to protect all the other passwords stored by Firefox. In this case, you will have to reset your master password. Note that doing so will cause Firefox to delete all your stored usernames and passwords. For instructions, see this article: Reset your Primary Password if you've forgotten it.

You may want to keep a backup copy of the two files key3.db and signons.sqlite in case you remember the master password.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Password_Manager