depende sa imo preference
firefox has tab browsing and secure
i also use IE 7 its now like firefox 2 but IE7 is now more secure.
Alot of exploits for firefox are coming out this days. it was IE7 that let the developers of FF to develop FF3 which is under development right now and MS is also working on IE 8. the 2 browsers almost share the same features now due to tight competion. that make us lucky.
For me. I use both. since different sites were optimized on either of the 2 browsers. if the page is not correctly rendered in FF, launch IE or vice versa.