Firefox is the most successful and widely used browser apart from Microsoft's Internet Explorer. It has so many features and extensibility capabilities that made Microsoft to rethink how a web browser should be.
When you enter a username and password in a web page, Firefox browser asks "Do you want Firefox to remember this password?". If you an user who says Remember then you should read this article to understand how unsafe it is.
When you ask Firefox browser to remember username and password, you at the high risk of exposing your credentials to others. It is very easy for any one to see all the usernames and passwords by following the menu option Tools -> Options -> Security -> Show Password.
Firefox not only reveals the usernames and passwords but also the web site associated with the login information. So never leave your Personal Computer unlocked that has Firefox browser.