Firefox, IE8 and Safari have all been successfully hacked at CanSecWest's Pwn2Own event.
IE8, Firefox and Safari Hacked to Bits at Pwn2Own : Read more
IE8, Firefox and Safari Hacked to Bits at Pwn2Own : Read more
I'm surprised tech companies don't hire professional hackers (or crackers, as the good guys like to be called) to find holes so they can fix em early.