If you have a lot of Firefox tabs open, your most basic ones (web mail, etc) wind up way over on the left, while your most recent ones pile up way over on the right. You can click on a tab and drag it so that your most important ones stay clustered together. Works on Mac and Windows.

Also, for the multiple-tab addicted FF user, a simple, though highly customizable add-in to check on is "colorful tabs"