The WiKID Strong Authentication System is a commercial/open source software-based two-factor authentication solution. We built WiKID to be a flexible, extensible, and secure alternative to tokens, certificates and passwords. The Firefox extension uses semantic web technology to automatically enter the one-time password for user, making it very simple for the end-users. The WiKID token supports multiple WiKID domains, eliminating the need for a token 'keychain'. Download the open source server or test the extension on our website
UPDATE: Instead of releasing a plugin, we have released an HTML5 software token that works in Firefox and many other browsers. Please visit http://www.wikidsystems.com/downloads/token-clients to see it in action.