All were running on Windows, except Safari which was running on OSX. You can see the details at this overly long URL from the contest organizers:
http://h30499.www3.hp.com/t5/HP-Security-Research-Blog/Pwn2O...
http://h30499.www3.hp.com/t5/HP-Security-Research-Blog/Pwn2O...
I wonder how the score would go if linux and bsd targets were present and if they allowed chrome and firefox on mac os x as well...