Rated 5 out of 5 stars
This addon in combination with Vertical Bookmarks Toolbar made it possible to get the bookmarks into a vertical bar.
Unfortunately Vertical Bookmarks Toolbar is broken on FF15 and the author doesn't want to maintain it any longer.
Maybe someone else would like to maintain it?
UPDATE: Thank you Gomita. I really like it the way it works now. You're the best! :-)
Try Vertical Toolbar 0.3 which displays Bookmarks Toolbar Items vertically.