I've been using Firefox for about two years now (I think). Part of it is personal choice but there are also other reasons. The ActiveX component in IE causes many security vulnerabilities. It's true that Firefox also has security problems but IE is more frequently targeted by hackers because of its larger user base.
The great thing about Firefox is the extensions feature. The program is set up to allow and encourage users to create modifications and additions called Extensions. Several of these are nearly essential including AdBlock Plus, Flashblock, Image Zoom, Paste Email, and QuickNote. Useful extensions include IE View, BBCodeXtra, and DictionarySearch.
The Session Saver feature is nice too. If the program crashes or you close it by using Task Manager, the next time you try to open the program, you'll see a pop-up message asking if you want to restore all the open tabs from the previous session. (It doesn't do this if you close the program normally.)
There are extensions for IE too but there aren't nearly as many and some of them require payments to download and use. Firefox was set up to create an online community of coders who would write these extension add-ons. All of them are open-source, so they are free and legal to download and use. I would only download extensions from the official Mozilla site, however. Extensions hosted on other sites may not always be compatible with all system set-ups. An exception is the McAfee Site Advisor, which is available for both Firefox and IE. After it is installed, it provides a small indicator next to each website in a Google search results list. Green means that it has been checked out as legitimate. Yellow means that the site may have some pop-ups and registration may result in some spam emails. Red means that the site could be dangerous to visit because of viruses, keyloggers, etc.