Security
Though some browsers are better than others at keeping you safe online, all of them have at least some security features. Phishing filters, for example, have become more or less universal over the past few years. These filters typically connect to an online database of known phishing sites to keep tabs on which sites are kosher and which aren't; when you browse to a known phishing site, you'll usually get a warning indicating that the site may be trouble. The downside to such filters is that they may not have brand-new phishing sites on their lists, so you'll still have to use your own judgment.
Usually browsers will also warn you before you open any applications that you download; the reason behind this, of course, is to keep you from getting caught flat-footed opening a piece of malware disguised as an image file, for example. Again, these features have limitations: They can't distinguish bad software from good software, so you'll still need a separate antivirus app for that.
Every browser we looked at has some form of private-browsing mode, as well. Though these features are useful for preventing the next person who uses the computer from knowing what you've been up to (you were purchasing a gift for your significant other on Amazon, say), they won't protect you from online security threats like phishing--so don't let your guard down.
Internet Explorer 8
Historically, Internet Explorer has suffered a bad reputation when it comes to security, but IE 8 has some solid security features in its own right. IE 8 displays sites' domains in a darker text color, so you can more easily see whether you're actually visiting an ebay.com page, for instance, or, instead, a fake eBay page on a phishing site you've never heard of. Microsoft could still put a little more emphasis on the domain name (using a different-color background, for example), but the highlighting is a welcome addition, and Google Chrome has since picked up this little trick. In addition, IE 8 provides a cross-site scripting feature that can prevent various types of cyberattacks.
Firefox 3.6
Firefox does a good job at keeping you informed about the state of your browser's security. For instance, when it automatically installs an update, it also checks to see if any of your plug-ins need updating as well, and it warns you accordingly. You can run this check at any time within Firefox by visiting the Plugin Check site. (Bookmark that page! Do it now!) The plug-in check seems to work in other browsers, too; but when I ran it in other browsers, it erroneously told me that I was using an up-to-date version of Flash when I wasn't (shame on me).
Another security feature: Firefox displays the name of sites that provide "identity information" in a box adjacent to the address bar. When you click on that box, more details about the site pop up.
Chrome 5
Google took a novel approach to security with Chrome: Each open Web page is "sandboxed." That is, if a site you open has been hijacked by cybercriminals, sandboxing can help prevent malware implanted on that site from accessing the files you have stored on your PC. If you're more interested in the technical nitty-gritty, see Google's blog post on the topic.
Safari 5
Though Safari has a fairly standard set of security features--phishing protection, private browsing, a "Reset" function--one particularly useful feature is its handling of browser cookies (small files stored on your PC that typically save preferences for a site). Instead of merely letting you enable or disable cookies altogether, Safari gives you a third option that enables cookies for the sites you visit but disables other cookies that are on the same page. For example, if you visit a news site, Safari will accept cookies for just that site, and not cookies for the page's advertisements.
Opera 10.6
Like Firefox, Opera is good at displaying the status of a Website in the address bar. It uses color coding in the address bar to indicate whether a page is encrypted (and if so, whether the page has any problems), or whether the site is flagged as fraudulent (Opera uses AVG's database of fraudulent sites and those carrying malware).
Similar to Safari, Opera lets you choose to accept cookies only from the site you visit (thus blocking advertising cookies, for example), but it allows you to do so on a per-site basis. To change this setting for a particular site, right-click the page and select Edit Site Preferences from the pop-up menu.
BEST SECURITY: Chrome
For security, we have to come down on the side of Chrome. Its page sandboxing is a great security feature (and one that you'll never notice), and as a result Chrome was the last browser remaining at this year's Pwn2Own hacking contest.