Vertical bookmarks with FF15 - no problem! :) Rated 5 out of 5 stars
Vertical bookmarks broken in FF15? No Problem!
Deinstall "Vertical bookmarks toolbar" and install "Vertical Toolbar 0.3" (or higher).
You can find the newest versions of Vertical Toolbar here:
Just drag the "Bookmarks Toolbar Items" into the vertical bar.
EDIT: All thanks to Gomita. He's the guy who fulfilled our wish. :)