Firefox allows users to save the username and password for a website. And when users access this website again, it uses the saved username and password for logging in to the site. In case if you have forgotten the username or password for a website then you can Firefox Page Info option to display the saved user credentials.
Access the website in Firefox for which you want to know the username or password . Click on the Tools menu –> Page Info
Click on the Page Info from the menu list.
Navigate to Security tab and click on the View Saved Password button. This would display the following Saved Passwords window with Username information for that site. If you want to know the password then click on the Show Passwords button.