Moved, please post in the correct forums. The tabs in Mozilla are a more convinient way of having many windows open at once. In internet explorer, you would have to have 3 or 4 items on the task bar for 4 different sites. In Mozilla, you can have many pages open at once, but it's only one item on the taskbar. It keeps navigation and the display neater.
Waffles's absolutely right. Not only that, but from a resource perspective, you only have one instance of the browser open, so less CPU/RAM usage.
Its also a lot easier to group similar sites which you want to keep in one window. I use them to preview changes on sites like blogging. I keep the preview in one tab while the dashboard in the other.