Firefox, like many popular browsers, includes a built-in functionality to save your password. Often we'll use the saved password feature so often that we've completely forgotten our password when we need to login to the same website on another computer. Here's how to locate your saved password.
In Firefox, navigate to the Tools \ Options menu item. Select the Privacy button, the Passwords tab, and then click on View Saved Passwords. You'll be presented with this screen:
Click the Show Passwords button, and navigate down to the website password you are looking for.