Firesheep came out on the scene in 2010 and rocked the world... and never made it to version 1.0. Mozilla has since released 15 major versions of Firefox and is incompatible with the extension. Is there a version that works with the latest Firefox or is one the 1000 forks on GitHub considered the successor to Firesheep?
Cookie Cadger, being developed by a colleague of mine. Terrifying Java app that steals cookies from a large number of apps:
Edit: Available now at https://www.cookiecadger.com/
To my understanding, all that Firesheep did was sniff out unsecured packets and pulled the information from there. Unfortunately there isn't a replacement extension (to my knowledge), but there are other options.