With LoginManager you can manage different accounts for different sites. You enter all information for a site once and then log in with just one click.
The site is loaded (in a new tab if you whish), the text and password fields are filled with the information you previously saved and the form is automatically submitted. All with only one click.
This extension is very useful if you have a lot of accounts on the same site and you have to re-login/change the account a lot.
Please read the "Developer Comments" for further information.