PFS is a property of an SSL connection which ensures that previously recorded encrypted traffic cannot be easily decrypted if the SSL private key later becomes available
for example, as a result of a court order, social engineering, an attack against the website or cryptanalysis.
When you visit a web site which uses SSL, the Extension will detect if it is likely that your web browser has negotiated an SSL cipher suite which supports PFS. It will display a green tick if so, and a red cross if not. Additionally, if the connection does not support PFS or is affected by Heartbleed, a warning triangle will be displayed on top of the Netcraft icon.