In the PC-Active of this month (a Dutch PC magazine) I read about the Tree Style Tab extension for Firefox. It moves the tabs so it can become a left hand column style of tabs (although you can drag it anywhere) AND it presents it in a tree format, which means that you have a much more ordered view on your tabs. It was created by Piro, who has made a lot of other Firefox extensions.
I prefer to have my tabs on the left hand column. So..I’m glad I could add this add-on to my Firefox essential add-on list
☞ after installing it absolutely try out the "options" (in the Add-on menu) since you will get dozens of options to make the thing work exactly as you like it e.g. give it the style you like. (I choose the metallic theme with auto-hide).
☞ notice that your brain has been trained to go the top of the screen, so it will take you a day to train your brain that it is now on the left.
p.s. if you use Windows and would like also your Taskbar items on the left hand side use TLB.