Mozilla’s Firefox web browser marked a milestone this past week, celebrating its fifth birthday.

No question about it, the open-source browser has been a big success, with growth that has been impressive by any measure. As of the end of July, Firefox had been downloaded more than 1 billion times.

Indeed, a mainstream site like this one here (WordPress) reports that Firefox now represents a larger share of activity than Internet Explorer — 46% versus 39% of traffic.

But now that Firefox has come of age, it’s facing some of the same “grown up” challenges that other browsers face.

In fact, application security vendor Cenzic has just released its security trends report covering the first half of 2009. Guess what? Firefox led the field of web browers in terms of reported total vulnerabilities. Here are the stats from Cenzic:

 Firefox: 44% of reported browser vulnerabilities
 Apple Safari: 35%
 Internet Explorer: 15%
 Opera: 6%

Compared to Cenzic’s report covering the second half of 2008, Firefox’s figure is up from 39%, while IE’s number is down sharply from 43%.

Welcome to reality. As Firefox has grown in importance, it’s gained more exposure to vulnerabilities. A significant portion of those vulnerabilities have come via plug-ins.

Mozilla is trying to take steps to counteract this, including launching a plug-in checker service to ensure that users are running up-to-date versions. It also offers a “bug bounty” to anyone who discovers security holes in Firebox.

And the good news is that even though Firefox had the highest number of vulnerabilities, even Cenzic admits that this doesn’t necesarily mean Firefox users are more vulnerable to security threats. Plus, those vulnerabilities tend to be patched more quickly than those found in other browsers.

So on this fifth anniversary milestone, Firefox can be justly praised as a major success story in the web browser world.