Verisign, the main digital certificates issuer, released some time ago the Verisign EV Green Bar extension, a Firefox extension that helps identify web sites featuring Extended Validation certificates. A digital certificate is a unique document a web site presents to a user's web browser as proof of its identity to start a secure connection (SSL or TSL). 

Extended Validation certificates are issued following a more comprehensive identity verification process as defined by the CA/Browser Forum guidelines which include (from Wikipedia):

• Establish the legal identity as well as the operational and physical presence of website owner;
• Establish that the website owner has exclusive control over the URL; and
• Confirm the identity and authority of the individuals acting for the website owner, and that documents pertaining to legal obligations are signed by an authorized officer.

The extension only function is to make it clear when a visited web site has an EV certificate. It turns the location bar green and adds the legal name of the company which can be clicked for more details about the certificate.

Mozilla is a member of the CA/Browser Forum and among other alternatives for increasing users' awareness about the legitimacy of a web site to help prevent phishing, is considering a similar behavior along with other improvements to the location bar.

As good as this may sound, the higher cost ($995/year for a regular certificate versus $1,495/year currently at Verisign) of the certificates and the exclusion of some kind of companies according to the guidelines could result on a disadvantage for small businesses if EV certs (the green bar) becomes a synonym of online trustworthiness. Also, some studies based on the Internet Explorer 7's implementation of EV-certs notification (which is almost identical to Verisign's extension) is not as effective preventing phishing attacks as thought.

You can try Verisign Green Bar Extension at Mozilla Add-ons.