On December 11, Canonical published details about Firefox vulnerabilities for its Ubuntu 13.10, Ubuntu 13.04, Ubuntu 12.10, and Ubuntu 12.04 LTS operating systems.
According to Canonical, Firefox could have been made to crash or run programs as the user's login if it opened a malicious website.
For example, multiple memory safety issues have been discovered in Firefox. If a user were tricked into opening a specially crafted website, an attacker could potentially exploit these to cause a denial of service via application crash, or execute arbitrary code with the privileges of the user invoking Firefox.
For a more detailed description of the problems, you can visit Canonical's security notification.
Users can simply fix the security flaws by upgrading their operating systems to the latest Firefox package specific to each distribution.
A normal system update, executed with the Update Manager, will implement all the necessary changes. A complete system restart is not necessary, but you will have to restart the application.
Alternatively, you can download Mozilla Firefox 26.0 right now from Softpedia.