The number of widescreen monitors are rapidly increasing, and vertical screen space can be a premium on such monitors. So, Firefox users usually try to move bookmarks and other elements to the sidebar. It is also beneficial to load pages of less importance in the sidebar.
Going by this trend, Vertigo Firefox addon moves the tab bar to a sidebar on the left, and stacks tabs vertically. This can be unyielding on regular monitors, but can be a boon for widescreen monitors. Stacking tabs vertically is also very beneficial if you are a heavy tab user, and open several dozen tabs at any given time.
The only downfall of using Vertigo is that it isn’t part of the sidebar, which means that enabling other sidebar elements like bookmarks or history can take decrease the screen space left for browsing (sidebar+vertical tab bar takes up too much space).