Trojan attacks mostly and some fraudulent websites such as faking a financial institutions website such as a bank. I looked into these problems at work where so of the users have windows 7 (About 80% now) where they all like to use the newest and greatest thing. Where firewalls that windows run or any anti virus firewall stop these intrusions before they are embedded on your computer none support 64 bit browsers. It does not matter what program that you use it appears. One user’s computer got a virus off a website that we use a lot at work no one else got this virus but this user. Well after removing this virus several times in one day we went through each website until the virus reappeared. I then went to my computer and opened two different browsers that where 64 bit and got the virus on my computer after that we disabled all 64 bit web browsers. This is just one case and I have seen this at least 5 times at my job. I had the same thing happen to me at home and now I stay away from them.
If you read the fine print on almost all of the major firewall companies they do not support 64 bit browsers at all. They barely support 64 bit. Even Microsoft support of 64 bit is sometimes nonexistent. I know that for the most part 64 bit browsers are new but with the problems I have seen I think I can say that they have major security flaws.