Actually, if you visit independent sites you will find different opinions.
For example, Secunia shows 77 vulnerabilites for IE7 and 67 for FireFox 3.x.
Considering the short time FireFox 3.x has been available I would say that
isn't a good track record. If your primary goal is speed then go with Chrome.
All have their strengths and weaknesses.
I'm OK with IE8 and it does what I need without a lot of third party
add-ons, but some people want open source at any cost and regardless of the
issues (FireFox 3.0.9, for example, fixed a bug that caused the loss of
cookies). Newcomers to the browser market (Firefox and Chrome) are welcome
and drive innovation.