"The flaw means that if you are viewing a trusted site in one window (PayPal or your bank) and open a site belonging to a spoofer in another window, the spoofer can insert code in the window showing the trusted site," wrote a moderator on Mozilla's online forum Monday.The same vulnerability was discovered almost a year ago in virtually every browser available on the market, but a slight variation of the fixed flaw is still present in Mozilla's browsers. More details at InformationWeek
Seven year old flaw found in Firefox

Security firm Secunia writes all Mozilla 1.7.x and Firefox 1.x browsers are vulnerable to a frame injection flaw that was first discovered seven years ago in 1998. The flaw allows malicious users to insert their own content into the view of trusted, legitimate sites: