Firefox users most likely to use secure browser
Security researchers from ETH Zurich, Google, and IBM Internet Security Systems published a report on web browser security. Here are some excerpts:

The study revealed that 637 million Internet users worldwide who use web browsers are either not running the latest version of their preferred browser or have not installed the latest patches.
Over the past 18 months, the study also shows, a maximum of 83.3% of Firefox users were using the latest major version of the web browser and also had all current patches installed (i.e. latest minor version). Only 56.1% and 47.6% of Opera and Internet Explorer users, respectively, were similarly utilizing fully-patched web browsers.
In June 2008, only 59.1% percent of Internet users worldwide use the latest major version of their preferred web browser. Firefox users are the most attentive: 92.2% of them surfed with Firefox 2, the latest major version before the recently released 3.0.
The study goes into many more details and makes some suggestions.
Further Reading:
Google: Are you using the latest web browser?
The Full Report: Understanding the Web browser threat
Otto - 7is7.com