Tabs have become a ubiquitous part of the web browser and it is hard to imagine using a browser without tabs anymore.
However, if you are using a widescreen monitor, then you might have noticed that most of the time when you browsing web sites, a lot of screen estate on the sides is being wasted. Here is where Tab Sidebar comes in handy. Tab Sidebar is a Firefox addon which once installed hides the original tab bar on the top and instead allows you to have tabs on the sides and since there is extra space on the sides, it even manages to display thumbnails of the pages. The extra pixels that you gain vertically allows you to view more of a page.
You could actually display the Tab Sidebar along any of the sides depending on your preference. By default Tab Sidebar hides the original tab bar but for some reason if you do want to retain it, you can do so in the Tab Sidebar options.