Most tech-savvy people are good about keeping software up to date. Or, at least, we'd like to think we are. Newer software is also good at keeping itself up to date, whether by automatically updating itself like Windows and OS X, or nagging users endlessly reminding them to upgrade like many A/V suites and some browsers do.
Despite those measures and increased user awareness in recent times, outdated web browsers are apparently a source of concern with nearly half of all deployed browsers out of date. According to a survey, roughly 45% of users on the Internet are using old versions of Firefox or IE, which puts them at risk for being exploited by security holes that may have long since been patched. I guess those naggings aren't enough.
They also cite Firefox 2 as being the most secure browser around but it didn't have anything to do with the number of exploits discovered on it versus IE, but rather how many users were up to date. According to them, around 83% of Firefox users are using the most current version, with only 65% of Safari users being up to date, 56% of Opera users and a mere 47% of IE7 users. The problem is twofold: Choice and knowledge. Some people don't update because they simply don't want to or have compatibility concerns. Others, however, probably don't even know that updating needs to be done.