Internet Explorer has some big memory issues. They may or may not fix this in IE 8, but hopefully they do. FireFox simply just uses a whole lot less memory than Internet Explorer. That is all I have to say about memory with these two browsers.
The Community
Believe it or not, the community plays a vital role. FireFox is backed by a huge community of volunteers, who code and design. Internet Explorer may have a community, but it differs a whole lot from the Mozilla FireFox community. A good example is when a vulnerability, or a bug is found for FireFox. They immediately fix the bug, and release an update for it. While the Internet Explorer "team" could take months, or even years to find the bug, then release an update or patch for it. This is one of the big reasons why FireFox is the safest.
Themes and Plugins
FireFox allows you to install Themes and Plugins for various uses. Internet Explorer still does not have this feature. FireFox is also Web Developer friendly with the plugins it offers, and the built in features that come with it. The IE team said Internet Explorer 8 will be more web developer friendly. I guess we will see, but don't hold your breath waiting for them to add any new features.
Web Standards
Internet Explorer does not follow web standards. This gives us web developers and designers headache, after headache trying to get our work compatible with Internet Explorer. This is especially true if you are dealing with Internet Explorers buggy CSS implementation.
Conclusion
Although I can not stand Internet Explorer, I am still a fan of Microsoft, and their products. If Internet Explorer wasn't one of the popular browsers than I wouldn't be writing this, since we wouldn't have to test our pages, because no one would use it. Since that is not the case I think Microsoft should wake up, and fix their browser!