Build a browser plugin that collects and saves user passwords

We need a browser plugin that does 2 functions: 1) Detects cookies from existing sites such as Facebook, Twitter, LinkedIn - if one is detected, it should say "You are logged in as 'username' - would you like to use this account?" 2) Lets a user manually enter their credentials, and then saves them so that when they click an application later it automatically logs them in. No need to enter credentials again. This will work much like Dashlane - [login to view URL] - but we do not need any desktop software, only a plugin that will interact with our website. We can start with just one browser (Chrome or Firefox) and do another browser at a later date.