How execute automatic login with Firefox?

Hi,
do exist a Firefox command that allow auto login?
When I enter in a site that ask user name and password, I wish that the program put automatically this information (it already do this when saving the password) and enter.
The result is that when I connect to the site for autentication, automatically do it, as saving cookie but some web site cancel cookie after short time.
I have already tested with failure AutoAuth, Autologin, LoginManager, Secure Login and Selenium.
Is there a solution to the snag?
Thanks