Everyone is used to access the tabs in Firefox just below the Firefox header that contains the navigation elements and address bar. While that seems to the the natural placement for now I remember times when tabs where accessible at the bottom of the screen just above the status bar instead.
And the Tab Sidebar add-on for Firefox tries to introduce another possibility by moving the tabs to the sidebar. This might have a positive effect on widescreen monitors that have lots of width. Many websites do not make use of a width wider than 1000 pixels anyway and those that use a dynamic width can adjust to different width anyway.
The main advantage of placing the tabs in the sidebar is the same why some Windows users prefer to move the taskbar from the bottom to the left or right side instead. The vertical alignment provides additional space that can be used to display additional information or more of the same kind. A vertical Windows taskbar for instance can display more folders with better information (folder names) than a horizontal taskbar can.
