winpcap(download) Firesheep(download) Follow the steps properly :
1. First of all download "Firesheep" from the above link and use the "openwith" option in the firefox browser.
2. Once you have installed firesheep on firefox web browser, Click on view at the top, then goto sidebar and click on Firesheep
3. Now click on the top left button "Start capturing" and it will start to capture the session cookies of people in your wifi network, This will show you the list of those people whose cookies are captured and have visited unsecured website known to firesheep, Double click on the photo and you will be logged in instantly
Remember Few Points
Install Wincap its Necessary else it won't capture data packets.
Make sure you are on Public Wifi with some active Facebook users :).
Run Mozila as Administrative if it does't work.