Firefox accounted for almost half of all browser vulnerabilities in the first six months of 2009, according to Cenzic.
The web security company claims Mozilla's browser had the largest percentage of web vulnerabilities over the six-month span, while Apple's Safari had the dubious distinction of coming in second. Microsoft's Internet Explorer (IE) was third, while Opera Software's flagship browser took fourth place.
The Cenzic report can be downloaded from the company's site (download PDF).
"It's not rocket science," said Lars Ewe, Cenzic's chief technology officer, referring to the browser bug counting. "We used several databases, including the CVE (common vulnerabilities and exposures) database to count the number of known vulnerabilities."
Firefox accounted for 44% of all browser bugs reported in the first half of the year, said Ewe, while Safari vulnerabilities came to 35% of the total. IE, meanwhile, accounted for 15%, while 6% of all the flaws were in Opera.
Cenzic did not separately count the number of 'zero-day' bugs - those unpatched at the time exploit code went into circulation - said Ewe, who defended his company's tally at the same time he downplayed their significance.
"At the end of the day, the number of vulnerabilities is only one measurement of a browser's security," said Ewe. "We're not trying to point a finger at any one browser. I would certainly not abandon Firefox because of this."
Ewe admitted that he uses Firefox for his personal browsing, and noted that Mozilla is "usually very fast to react to bugs".
Mozilla has been slammed for the number of flaws it fixes in Firefox before. Last spring, for instance, Jeff Jones, a director in Microsoft's security technology unit, and Mike Shaver, the vice president of engineering at Mozilla, traded barbs about browser security after Danish security vendor Secunia published a report that said Firefox had nearly four times as many flaws as IE during 2008.
As far back as December 2007, the companies sparred over bug counts after Jones claimed IE had been affected by fewer than half as many vulnerabilities in the previous three years as Firefox.
In those instances, Mozilla has defended itself by arguing that it patches vulnerabilities significantly faster than Microsoft, and that its open-source approach means it doesn't hide flaws other vendors may fix in undercover updates.
For his part, Ewe blamed Firefox's high vulnerability count on the browser's rising popularity. "Firefox clearly has some a momentum," he said. "When you gain momentum, you're exposed more [to security researchers and hackers]."
Ewe said that Safari, which controlled just 4% of the market last month, accounted for 35% of all browser vulnerabilities because of a large number of reported flaws in the version that runs on Apple's iPhone.
But Ewe would rather have users focus on browser security overall than dwell on which application has more flaws, fixed or not. "In general, you can make the argument that all browsers have room for improvement," he said. "They have to choose between usability and security, and user-demanded behavior that makes them choose usability over security. That being said, all are trying to be better."
Compared to two years ago, or even a year ago, browsers are more secure now than ever, Ewe said. "Definitely, everyone's made progress."
Data from web metrics company Net Applications puts IE as the most-used browser, with about 65% of the market, followed by Firefox with 24%, Safari with 4.4%, Google's Chrome with 3.6% and Opera with 2.2%.
Mozilla did not reply to a request for comment about the Cenzic report.