Neil Deakin writes about 101 things that you can do in Mozilla which you cannot do in Internet Explorer. The first 5:
1. Tabbed browsing: Lets you display more than one site in a window using multiple tabs.
2. Popup blocking: Block all those popup ads.
3. Prevent scripts from doing various things such as moving windows, closing them and setting cookies.
4. Link toolbar: Displays content from the tags provided by a document, allowing one to navigate to various parts of a site.
5. Sidebar: The sidebar provides a number of tabs by default, and others can be added by the user.