Of the dozen flaws fixed in Firefox 3.0.9, four were rated "critical," two "high," two "moderate" and four "low" in Mozilla's four-step ranking system. It was the most vulnerabilities Mozilla has patched since December 2008, when it quashed 13 bugs.

The four critical vulnerabilities -- two in the Firefox browser engine, two in its JavaScript engine -- were patched by a single multi-fix update that Mozilla, as is its practice, said might be exploitable. "Some of these crashes showed evidence of memory corruption under certain circumstances and we presume that with enough effort at least some of these could be exploited to run arbitrary code," the boilerplate text said in the .

Other patches prevent from sending users to malicious Web sites and to conduct cross-site forgery request attacks or secretly plant cookie-like objects on a machine to track the user's movements on the Web.

Mozilla addressed several non-security bugs in the update as well, including a database corruption problem that deleted cookies and an issue with on some Web-based services, including AOL's e-mail and instant messaging services.

Tuesday's update, the third this year, follows by four weeks an emergency patch Mozilla hustled out to fix a bug that had in the Pwn2Own hacking contest.