I never really though about it that much, but I learnt today that you can actually reorder the tabs in Internet Explorer 7+ and Firefox by clicking and dragging the tab headers. This is one of the features that probably most people don't use much, it isn't advertised as such, and yet fits naturally into most people's expectations of how it should work.
According to Evil Tester, there are some issues and differences between the two browsers. Makes for some interesting reading.
If you would like something similar to this for Windows, then check out Taskbar Shuffle.