Very buggy and needs work! Rated 1 out of 5 stars
If you want to toggle the add-on bar, it is built in to Firefox. The add-on does more than that, but the arrow disappears for me when it is minimized. I even rearranged my icons to resemble the author's but it still doesn't show the arrow when minimized even after resetting the minimum width.
As a previous commenter said, the scrollbar is hidden at the bottom. This is annoying, and horrible if your window manager places both arrows at the bottom. I would suggest putting it on the left instead of the right by default with an option to place it on the right for window managers that place the scrollbars on the left.
Adding a combined minimize and restore button with a maximize button that is visible when restored to reset the add-on bar to the default location and width would make this idea work.