Generally, Which is not possible to open more same accounts(Gmail, Yahoo, Hotmail, Facebook etc..) like email, social networking logins in a single browser. Which is due to cookies and other issues. However Google Chrome brings excellent way to open more accounts in a single browser.By doing that, open a facebook login in Google Chrome and then for next account, right side top corner there is a key icon click and select -> New incognito window, now it comes with new window, here you can login another facebook account.
According to these procedures you can login gmail, yahoomail, hotmail,twitter etc..