Turn Firefox to Always Auto Save Password Without Showing Notification Bar

1. Close Firefox
2. Edit nsLoginManagerPrompter.js with notepad which is normally located in C:\Program Files\Mozilla Firefox\components\
3. Replace the entire lines 642 to 711 with the code below:

   var pwmgr = this._pwmgr;
   pwmgr.addLogin(aLogin);
   

Now whenever you login to any website, Firefox will auto save the site, username and password to the login manager WITHOUT showing the notification bar.

Note: even when you enter the wrong username or password, it will still be saved.

Source

http://www.superuser.com