A regression error occurs when the programs process certain IMG tags. Attackers who successfully lure users to a malicious Web page could then exploit the flaw to bypass restrictions and run arbitrary code.
The flaw specifically affects Firefox version 184.108.40.206 and 220.127.116.11; and SeaMonkey 1.0.7.
Users will be protected from the flaw by upgrading to Firefox 18.104.22.168 or 22.214.171.124; or SeaMonkey 1.1.1 or 1.0.8.
Mozilla released those versions last week to fix more than 10 other Firefox flaws digital miscreants could exploit to circumvent security restrictions, conduct cross-site scripting attacks and access sensitive information.
Mozilla 2.0 has suffered from a variety of flaws since its release last October.
Mozilla security chief Window Snyder said in a recent interview that Mozilla tries to issue a security upgrade every six weeks or so.
"We're continuously looking for vulnerabilities and continuously fixing them," she said at the time. "Users don't have to wait for the next version of the product to get a lot of the benefits of the security work we're doing. They get it on a regular basis."
She made that comment after being asked if the frequent security updates are an indication that the open source browser isn't as ironclad as supporters boast. Firefox is often touted by fans as a more secure alternative to Microsoft's much-attacked Internet Explorer.