According to a recent blog post by Secunia a security research firm Firefox users are the most updated and, consequently, the least vulnerable among web browser users:

Comparing browsers and looking at Firefox, Opera and Internet Explorer, we found out that Firefox 2 is the least vulnerable, as only 5.19% of all Firefox 2 installations miss security updates, whereas 11.96% of all Opera 9.x installations miss security updates, and the numbers for IE6 and IE7 are 9.61% and 5.4% respectively. These numbers are not that alarming and show that users are fairly concerned about applying relevant updates for their browsers – which naturally is one of the most exposed applications.

A quick look at Mozilla Links statistics reflect similar results: 95.17% of Mozilla Links' visitors in May using Firefox 2.0.x are already using the latest 2.0.0.3 release. Users in the 1.5.0.x branch are not as current: only 84.8% is using the latest 1.5.0.11 version. Firefox 2.0.0.x users make 90.7% of all Firefox visitors, 8.03% are in the 1.5.0.x branch and the rest in some even older version.

The results are in large part due to the Firefox Update System efficiency, which in my experience is the most intuitive, non intrusive and automatic I find among my currently installed software. Most of them don't even bother to announce an update, others are too shy to remind more than once, others just won't shut up, and others dare to call a 20MB download an update.