You might want to think twice before logging into Facebook and Twitter over an open WiFi internet connection. There is a Mozilla Firefox add-on Firesheep that allows cookie data to be revealed in clear text so to capture the user name and password which can lead to a HTTP session hijacking .
Using HTTPS for these sites are more secure.
There are some Firefox extension like Force-TLS, HTTPS everywhere allows your browser to change HTTP to HTTPS on sites that you indicate in the Firefox Add On “Preferences” menu, protecting your login information and ensuring a secure connection when you access social sites. Blacksheep is an another add-on which detects Firesheep Use on Wireless Networks.