Well, sure it is. According to the Firefox web site, which must of course be untainted by marketing claims since it is Mozilla, " Firefox continues to lead the way in online security".
OK, marketing hyperbole aside, I'm a data guy. I care about what the data says. Fortunately, Jeff Jones collected the data and did the analysis. Rather than color your conclusions by mine, I will let you draw your own conclusions from his analysis because (a) Jeff is a friend of mine and I won't let that influence a judgement, and (b) there may be a slight conflict of interest in the analysis due to Jeff's current employment situation. Nevertheless, it is an interesting read, and you can check the numbers for yourself.
Don't forget too that IE 7, under Vista, runs in low integrity, rendering a lot of attacks far less severe. Jeff forgot to mention that in his analysis. Firefox does not work in low integrity; at least not yet.