Robert Graham from Errata Security tested Force-TLS and found that it does not protect against Firesheep.
First of all, the plug-in “Force-TLS” does not protect you, as some have suggested. I proved this with Twitter, where I was able to sidejack the connection with both FireSheep and Hamster. I’m not sure what Force-TLS does, but it doesn’t force a connection to be TLS/SSL. I configured *.twitter.com (the domain and all subdomains), and the URL “http://twitter.com” still appeared in the address bar.
In addition, Firesheep’s ability to successfully sniff traffic depends on your network adapter.
FireSheep works only as well as the underlying packet-capture. On a Macintosh, the adapter can be fully promiscuous, capturing everybody’s traffic on the local access-point. On Windows, some adapters (like Broadcom) will see all the traffic, others (like Intel) will only see your own traffic (useful for watching which of your own websites can be sidejacked, but not useful for sidejacking others).
Rob provides extensive details and screenshots on his test methods.
