The U.S. Computer Emergency Readiness Team is advising people to upgrade to the latest versions of the Firefox Web browser and the Thunderbird email program to plug numerous critical security holes.

Issued this week, the warning from the agency within the Department of Homeland Security said failing to use the latest versions would leave computers open to malware that could enable an attacker to commandeer a PC. US-CERT said some of the vulnerabilities involved the way Firefox and Thunderbird handle URLs or images.

"By taking advantage of one or more vulnerabilities in Mozilla products, an attacker may be able to take control of your computer," US-CERT said.

Last week, Mozilla Corp., maker of the open source browser and email client, updated Firefox to patch two-dozen vulnerabilities, most of them critical. Firefox 1.5.0.2 debuted just days after rival Microsoft Corp. fixed 10 security problems within Internet Explorer.

Mozilla also has released fixes for Thunderbird and for the Sea Monkey browser suite, the replacement for the now-defunct Mozilla suite.