I saw this article that caught my eye yesterday;
http://it.slashdot.org/article.pl?sid=08/12/15/1424239
which references
http://www.info-svc.com/news/2008/12-12/
Basically it is comparing how browsers handle your passwords badly.
Anyhow, they score pass/fail on certain tests, IE7 got a 5, Firefox got a 7. They did not test IE8 beta2 which is more comparable to firefox 4.0.3.
The interesting thing is there is not a big difference in the case of passwords, and I suspect that is true for most cases. Firefox is slightly better, not hugely better. And guess what? You can run firefox all you want and you are not getting rid of IE on a windows machine. Microsoft chose to depend on IE for certain functionality in windows, and while you can make it not the default browser, you are not going to get rid of it and have a fully functioning OS. So you are basically only mitigating circumstances where your browsing habits expose the browser to a virus. You do nothing to prevent browser problems unrelated to what web sites you visit. If you want to avoid viruses, be a smart computer user and do not click every 'free smiley' offer you run across. Some people claim you need to have a soft and hard firewall to avoid viruses. That's crazy. I have been surfing since 92, basically when the internet became available to consumers, using windows, never owned an anti-virus program other than what came with windows, and have never had a virus.
Using firefox also makes you less of a target. Virus writers do not put the effort into attacking a small portion of the surfers. Up until very recently, Firefox (and apple etc) were less the 10% of the targets and no one bothered. But that is changing. Just last week Apple began advising users to run anti virus, and then quickly retracted the recommendation after they remembered it was a chief advertising tactic against Microsoft. But if Apple and Firefox's usage number continue to grow I expect you will see more equal attacks in terms of numbers against both IE and Firefox.
Bottom line; yes firefox is more secure. Is it a huge difference likely to affect you? Depends on how smart you are about using your computer. As far as feature set I do not see much difference between IE8 and firefox. If you are a casual user you might be a little better off with firefox, for a while.
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