"Because it isn't IE" isn't a sufficient reason. If you've read my posts, you'll notice I haven't said anything of the sort. I'm not a Microsoft hater.
So, why use Firefox?
1: Firefox is more secure than IE.
-Firefox comes with a pop-up blocker included. "So what? WinXP Service Pack 2 installs a pop-up blocker into IE!" That's true, except that its pop-up blocking is a less-robust implementation, and is merely a stopgap measure to keep you somewhat placated until IE 7.0 is released... in 2006.
-Firefox simply is not vulnerable to most of the various browser hijacks and exploits that are just waiting for IE, simply because the traps in question are written to take advantage of IE's ActiveX scripting engine, something Firefox lacks. No ActiveX means a webpage can't use ActiveX to load viruses and worms into your browser. "So what? A webpage can just pretend to install an XPI-format extension, when it's really a browser hijacking!" True, but only if you allow it. By default, Firefox is configured to only allow XPI installations from update.mozilla.org, Mozilla's official updating service. Any other page is automatically blocked from installing software. You have to manually add the page to a whitelist, and even if a malicious page somehow makes its way on to your whitelist, you must manually approve the installation of the software.
-The argument that Firefox will get hacked as often as IE if it becomes popular is a fallacy, because you presume that there is no inherent difference between IE and Firefox. This is wrong. Firefox is being continually improved (unlike IE, which has effectively halted development, barring the flood of security patches), and like I mentioned before, any new vulnerability is patched and a new release issued before hackers have time to implement the vulnerability.
-Firefox has less security vulnerabilities than IE. This is a fact. Firefox was coded from the beginning with security in mind: IE was not. Any exploits or bugs found in Firefox are often fixed and a new release issued the same day or the next day--if they exist at all. With IE, it's a near-certainty that you'll be downloading some kind of security patch every couple of weeks--whenever Microsoft gets around to issuing a patch. As a aside note, there are high-profile vulnerabilities built into IE that Microsoft has been aware of for years, yet has not fixed. What does that tell you about their dedication to keeping your computer safe?
2: Firefox is customizable.
-Firefox comes, by default and design, with only the bare essentials. These bare essentials are already more than what you get with IE: tabbed browsing, a Google search bar (customizable to include other search engines such as Yahoo! and Altavista, if you wish) built in to the browser, and Live Bookmarks (for RSS feeds.) But if and when you wish to add more features, such as an extension which will give you more options related to controlling your browser tabs, you need only go to update.mozilla.org and download the appropriate extension. And by the way, these extensions are secure and will not conflict with each other, unless you download some type of early beta (which, if you stick to Mozilla Update, will not occur.) You can also download themes to customize the browser's appearance.
3: Firefox is reliable.
-Firefox crashes less often, if ever, than IE. Firefox renders pages according to W3C standards, meaning that web pages look exactly how they were intended (unlike IE, which uses its own set of standards.) Firefox uses less system resources than IE, putting less of a strain on your PC. Firefox often renders webpages faster than IE, as well.
4: Firefox is migration-friendly.
-If you're switching from IE to Firefox, Firefox will ask you if you wish to import your settings, cookies, and bookmarks when you run it for the first time. It's a seamless transition.
To summarize, Firefox is preferable to Internet Explorer because it is not the computer programming equivalent of opening all your windows and leaving the door unlocked when you go on a two-week vacation. Firefox gives you more control over how you browse, ranging from the organization and cleanliness offered by tabbed browsing to the power of extensions. There really is no reason to use IE, unless you either need to access Windows Update, or you come across a page which will not allow you to view it with anything but IE (which can be worked around using an extension called User Agent Switcher, which identifies your browser as IE and not Firefox, thus granting you access.)
Get Firefox.
Or if you're still not convinced,
a quick Google search (performed from the Google search bar built into Firefox) reveals dozens of pages listing reasons to switch.