A great start.... Rated 3 out of 5 stars
With widescreen monitors being so common vertical screen real estate has became much more valuable & horizontal much more disposable, thus I've felt for awhile now that firefox should offer in the customize toolbars option where to display the tabs other than simply just "on top" (like google's chrome browser) or "on top" where firefox has always displayed them by default & should include the ability to choose vertically on either side of the display or even on the bottom, to keep them separated from the bookmark toolbar, if one has chose to have it displayed (I hate when rearranging my tabs I accidentally drop 1 somewhere in my bookmark toolbar).
This addon offers the ability to have your tabs listed vertically on either side of your display.
My only 2 complaints with the addon is that it does not integrate your persona. I'm using the persona "glass - blood" which is simply a solid shaded red. When using this addon the background of the tab list is completely white, totally ruining/breaking the visual appearance of the browser.
I have not tested it with other personas or themes so I don't know if this is true for all or just the one I am using.
The other complaint being that it attaches itself to the "sidebar", meaning that if you choose to have your tabs listed vertically on the right side of your display & you open either your history or bookmarks sidebar it will be on the right side as well. The tabs list & history/bookmarks sidebar cannot be separated. I would prefer to have my bookmarks sidebar be on one side & my vertical tabs on the other.
Other than those complaints it is an excellent addon. Unfortunately those 2 complaints are important enough to me that I don't use this addon. I can't tolerate the disregard for browser's persona... that's actually reason enough for me. I could tolerate the attaching itself to the general sidebar but I can't tolerate a broken color scheme.
A great start of an addon with room for improvement. I will be monitoring this addon in the hope that perhaps in the future it will improve in those 2 areas of which I complain.