i think it depends upon you.How much security measures you take while browsing. IE 7/8 can be safe too. If you say FF is safe. Let me tell you Mozilla regularly updating security flaws in FF, same thing is with IE too. FF is free basically and more customizable, that;s why it is getting popular. More you Customize the FIREFOX, more memory will it take. More over, some extensions are also use virus ways to access ur pc. FoxTorrent Extension is one of such examples.If I am wrong somewhere, do let me know. It wud help me to update myself.