Mozilla has released updates to its Firefox browser and Thunderbird e-mail client that fix a number of critical security flaws in the open-source products.The Firefox update, which was in the process of being automatically delivered to Firefox 1.5 users Friday, addresses 12 security flaws. The release also includes some fixes designed to make the browser more stable, Mozilla said on its website.Five of the browser flaws are serious enough that an attacker could possibly use them to run unauthorized software on an unpatched system, Mozilla says. Security vendor Secunia rates this update as "highly critical," its second-highest rating of importance.There are a couple of mitigating factors, however. First, hackers must still develop code that takes advantage of these flaws before users can be attacked. Second, a user would need to be tricked into visiting a malicious website for the attack to occur.Eight bugs have been fixed in Thunderbird, one of which is considered critical by Mozilla.More details on the Firefox vulnerabilities can be found here.Descriptions of the Thunderbird flaws can be found here.-Robert McMillan, IDG News ServiceFor related news coverage, read Microsoft Flaw Affects Windows XP, Windows Server 2003.Keep checking in at our CSO Security Feed page for updated news coverage.