While Internet Explorer was developed to wipe out Netscape from the market by overloading the product with features, Firefox has a very solid design and new functionality could be provided by third parties using a built in feature known as "Plugins", which is software added by users as they wish.
Here are the top 5 reasons why you should be using Firefox:
1. Respect of Standards. Firefox works following the standards dictated by the W3C Consortium, meaning that all pages built with those standards should be shown exactly the same in all browsers that respect those standards. Microsoft included functionality with the intention to keep users from using other browsers, which is felt as "bad behavior" by the programming community in general.
2. Improvement doesn't stop when they win. When Internet Explorer began his life in 1,995 nobody expected that it would threat the dominant position of Netcape, which had more than 90% market share. Microsoft worked very hard on Explorer, adding more and more features (and creating serious security risks because of the very fast developments) every year and suddenly, all developments stopped in 2,001 with Internet Explorer 6.0, till recently. This shows that Microsoft only moves when they are at risk.
3. Firefox is well designed for security. Internet Explorer is so insecure that even the CERT (Computer Emergency and Response Team) a high level security organization from the government, told people that they should be using anything but Internet Explorer. The problem is that Internet Explorer allows the execution of programs and this is an extremely high security risk. You can read the article here: http://www.securityfocus.com/news/8998
4. Commitment. The Mozilla Foundation is constantly developing never programs and solutions for users, regardless of market share or earning because all their products are free. They created Thunderbird (to read email), NVU (a web page editor) and many other useful programs.
5. Totally Free. The Mozilla Foundation develops all their software in the open source model, meaning that anybody can see the source and change it to their own needs. This helps in the creation of a much secure product because more eyes are looking into the possible problems.
I can give you many reasons more, but security is the best one. Firefox is much more secure by design while Internet Explorer has to be patched year after year, creating a more unstable product through time unless a complete redesign and rewrite happens, which is very unlikely. Why wait until Microsoft starts doing things well? Instead, get Firefox now and enjoy a faster and a more secure browsing.