This should be built-in! Rated 4 out of 5 stars
I've really enjoyed using Vertical Toolbar, which should've been part of Firefox by default ever since wide screens became the norm. Even though I use a 23" 1920x1080 LCD, switching just 1 toolbar to vertical made a noticeable difference, so I imagine it'd be excellent on a small screen.
I'd give it 5 stars, but there were a couple of problems. The small one is that it made Firefox ignore my theme & use the drab default icons. Much worse, putting my Bookmark Toolbar Items (it's 1 folder named .) onto V.T. made the "customize toolbar" menu option grayed out; the only way to fix it was to restart Firefox with Vertical Toolbar disabled.