Bookmarks, History, bookmarked web pages and every other content you may have set to be displayed in the sidebar through extensions can be moved to the opposite side of your window if it better suits your browsing style.You will need to open userChrome.css file located in your profile folder, or create a new one by renaming userChrome-example.css. Then add these lines:

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