When Firefox 13 was released to the public earlier this month, it came with an updated 'New Tab' page that seems to take inspiration from Chrome and Opera by providing thumbnails of the sites you have previously viewed. There is also a 'Tabs on demand' which aims to speed up your browsing experience.
Obviously for all its testing and quality control, at least one unintentional feature slipped through, in that secured content is easily accessible to anyone that is using the browser through the 'new tabs' page. Firefox 13 takes a snapshot of recently visited sites and this includes sites that were accessed over HTTPS used for secure communication to websites such as online banking.
In a report over at The Register Reg reader Chris discovered the feature after opening a new tab only to be "greeted by my earlier online banking and webmail sessions complete with account numbers, balances, subject lines etc."