The Most Popular is IE because the main reason is it is installed on all machines as the default. Also most basic users do not know of any other way to use the web. After IE the most popular is Chrome and then close behind it is Firefox. with that said let's move onto the important aspects of the browser.
Boot Time is how fast will it load when you select the button to open. all open within 1 second however the fastest is Firefox. Firefox is .o2 seconds faster than the other 2. Both Chrome and IE are about the same in this area.
Memory Usage is also a big concern, how much does it slow your computer down once it is open and while you are using it. Firefox is also going to be the best one here. Firefox runs at a 400KB usage where as the other 2 are right around 600KB.
Now for the most important one. Security, is or should be the most critical one of all. Again Firfox blows both out of the water clearly here. Second to Firefox is Chrome however there is some significant concerns between the 2 and then there is the worse one by far which is IE.
So the clear winner is Firefox. However Google has so much of the market in other areas as well as popularity it still remains to be the front runner. So beware all what seems to be the most popular is not always the best choice. For a common user or a developers standpoint, anything you get with chrome you can get with firefox.