Just my personal opinion, but my 'computing environment' seems less cluttered with just one FF icon on the taskbar instead of numerous IE icons. In its standard installation FF still opens other windows when prompted to do so by websites, you can circumvent this by installing the 'tabbrowser preferences' plugin from the Mozilla site.
The technical reason for liking tabs is that every new IE window is another instance of the complete program running, which drains resources. With multiple tabs open in FF you are still running only one instance of the program, keeping resources free for other applications.