Mozilla Firefox is an open source web browser from the Mozilla Foundation in the U.S.A. Mozilla Firefox ESR is an extended support version of Firefox (the web browser).Mozilla Thunderbird is email client software that supports the IMAP and POP mail protocols as well as the HTML mail format. A security bypass vulnerability exists in multiple Mozilla products that is a security limitation due to an error related to a specific HSTS configuration. An attacker can exploit this vulnerability to conduct HSTS attacks on subdomains.