Mozilla isn't actually a web browser - it's a platform. It supports writing UIs with XUL that incorporate different kinds of components (using XPCOM, which is cross-platform!).
One of the things it implements is the Navigator, which uses the Gecko (aka NGLayout) HTML rendering component. But, of course, this is not the only thing that has been implemented for the Mozilla platform. Some examples:
...and more, including completely outrageous projects like MozQuake... A nice list of different Mozilla applications can be found from http://www.mozdev.org/projects.html.
If Mozilla's automatic installation is turned on, adding applications to Mozilla is very easy - just click on download link, confirm, and it gets downloaded and installed.
Some reasons why I like Mozilla a lot:
- Bookmark keywords. Zillions of search engines effortlessly on the location bar! (Yes, works with E2 too. Detailed in E2-related browser tips)
- Tabbed browsing. Helps in X11 a bit, and a great deal in Windows.
- Gestures and - this is great - pie menus as a recent addition, both from Optimoz at mozdev.org - Great things, great things!
- "Lo-Fi" theme. You can't get Opera, or even MSIE, to look this neat and uncluttered with same amount of money invested =)