"The flaw derives from Firefox's willingness to supply the username and password stored on one page on a domain to another page on a domain. For example, username/password input tags on a Myspace user's site will be unhelpfully propagated with the visitor's Myspace dot com credentials."Using Control + Shift + Delete will clear private data in Firefox. In addition, turn off the Password Manager and the Firefox extension "Master Password Timeout" if you use it.
See Bugzilla Bug 360493