HI
IE 8 are right now the most safety browse, Firefox have some safety hole that can be used to hack you PC.
and if you shall use windows update you most use IE firefox do not work with that
so one day IE are most safe and firefox are that the next day, but to be totaly safe try Opera, www.opera.com
and if you also install Spybot - Search & Destroy then you are more safe
TJK