Opera came 1st in Cenzic report
Opera came tops in that 'security analysis' and even they dismissed it as tosh:
http://my.opera.com/haavard/blog/2009/11/10/cenzic-security
There's no published methodology. Everything points to them simply counting the number of 'vulnerabilities' that have been fixed. So Firefox, being open source and publishing everything comes out tops, while I.E. which only has to count the vulnerabilities found by others which Microsoft are then forced to (eventually) patch.
You can assess your security risks off of that report if you wish. I'm just glad you don't work for me.