But the security specialist also finds that Microsoft's browser is the only one widely exploited by hackers today.
But the report, released Monday, also found that hackers are still focusing their efforts on IE.
The open-source Mozilla Foundation browsers, such as the popular Firefox, have typically been seen as more secure than IE, which has suffered many security problems in the past. Mitchell Baker, president of the foundation, said earlier this year that its browsers were fundamentally more secure than IE. She also predicted that Mozilla Foundation browsers would not face as many problems as IE, even as their market share grows.
Symantec's Internet Security Threat Report Volume VIII contains data for the first six months of this year that may contradict this perception.
According to the report, 25 vendor-confirmed vulnerabilities were disclosed for the Mozilla browsers during the first half of 2005, "the most of any browser studied," the report's authors stated. Eighteen of these flaws were classified as high severity.
"During the same period, 13 vendor-confirmed vulnerabilities were disclosed for IE, eight of which were high severity," the report noted.
The average severity rating of the vulnerabilities associated with both IE and Mozilla browsers in this period was classified as "high", which Symantec defined as "resulting in a compromise of the entire system if exploited."
The Mozilla Foundation did not immediately respond to requests for comment.
Symantec reported that the gap between vulnerabilities being reported and exploit code being released has dropped to six days on average. However, it's not clear from the report how quickly Microsoft and Mozilla released patches for their respective vulnerabilities, or how many of the vulnerabilities were targeted by hackers, though Microsoft generally releases patches only on a monthly basis.
Symantec admitted that "at the time of writing, no widespread exploitation of any browser except Microsoft Internet Explorer has occurred," but added that it "expects this to change as alternative browsers become increasingly widely deployed."
There is one caveat: Symantec counts only those security flaws that have been confirmed by the vendor. According to security monitoring company Secunia, there are 19 security issues that Microsoft still has to deal with for Internet Explorer, while there are only three for Firefox.
The report also highlighted a trend away from the focus of security being on "servers, firewalls, and other systems with external exposure." Instead, "client-side systems--primarily end-user systems--(are) becoming increasingly prominent targets of malicious activity."
Web browser vulnerabilities are becoming a preferred entry point into systems, the report stated. It also highlighted the trend of hackers operating for financial gain rather than recognition, increased potential exposure of confidential information, and a "dramatic increase in malicious code variants".
Tom Espiner of ZDNet UK reported from London. CNET News.com's Joris Evers contributed to this report.