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
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

Note: For this tweak I recommend using the ChromEdit Plus extension.

The Firefox sidebar can display bookmarks, history and other content. Some users may prefer to have the sidebar displayed on the right side of the screen instead of the left. Simply add these lines to your userChrome.css file located in your profile folder:

/* Display the sidebar in opposite side */
#browser {
-moz-box-direction: reverse;
}

When you are done editing your userChrome.css file, close and restart Firefox and the changes should take effect right away.

Source