In the future, when you log in to the website you won't be prompted to save the username and password. To tell Firefox to never remember usernames and passwords for the current website, click the drop-down menu and select Never Save.To save the new username and password, click on Update. If you've stored the wrong username or password, just type the right one into the website and Firefox will prompt you to save it.The next time you visit the website, Firefox will automatically enter your username and password for you. To have Firefox remember your username and password, click on Save.If Firefox does not prompt you to save passwords, see the article Usernames and passwords are not saved.

To bring it back, just click the key icon on the left side of the address bar. Tip: Clicking outside of the Save prompt will hide it.