Moving The Sidebar
How to move the sidebar from the left side to the right side of the screen

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Tweak Type:	Display |Tabs
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

If you want to be different and have your tab bar at the bottom of the window, it can be done. You will need to modify your userChrome.css file (best if done with ChromEdit Plus) by adding:

/* Display the tabbar at the bottom */
#content > tabbox { -moz-box-direction: reverse; }

Source