Just to expand on what mirza said,
What really sets firefox apart form the others is the extensive library of 3rd party plugins available. With them, I can achieve a level a security not possible with other browsers. Here’s some of the plugins I use:
1) AdBlock Plus – blocks almost all ads, with block lists updated automatically.
2) NoScript – Run with javascript and flash disabled, with the ability to allow javascript/flash to run on a site by site basis. Can even activate one embedded flash app without activating everything on a page. Easily controlled by a icon in the status bar.
3) CookieSafe – Site by site control of cookie content. Easily controlled by an icon in the status bar.
4) Secure Login – prevent firefox from auto filling in username/password fields until you click on them.
5) Greasemonkey. Ability to run local javascript on a page after the page is loaded. With this, I can literally change how a website functions to some extent. It’s great for adding little features to a site.