if i'll use any of these email id's to sign in to my gmail account gmail will let me in to the account firstname.lastname@example.org because gmail ignores .'s(dots) in your id. but many websites(not facebook)treats them as different id and you can access multiple accounts in a website through one email id.
This trick is only for gmail accounts and will not work on yahoo and hotmail etc. To know how to access multiple gmail accounts at same time on same browser click here