The Mozilla Firefox browser is the most vulnerable browser in terms of malware and exploits, according to a study by applications security group Cenzic, with 44% of all browser vulnerabilities reported in the first half of 2009.

Firefox’s vulnerability score hit up a notch from last year’s 39% (closely following Microsoft Internet Explorer’s 43%). The current study gauges Internet Explorer at a 35% score, Apple’s Safari with 15% and Opera with only 6%.

“It’s a combination of different things,” says Lars Ewe, the CTO of Cenzic, in an InternetNews.com report. “… [but] a fair amount of the vulnerabilities have come by way of plug-ins,” Ewe continued, referring to Firefox’ popular open-source plug-in support.

“The plug-in architecture that they have is a selling fact for the browser and one of the reasons why I love using it,” Ewe shared. “They can’t control security aspects of all the plug-ins and the vulnerabilities are a side effect of that.”

Mozilla, however, seems to be aware of this fact. Just recently, Mozilla has released a plug-in checker service to check for the validity of a user’s plug-in versions, and the Firefox 3.0.9 update specifically addressed a significant number of plug-in vulnerabilities.