I've loved this add-on for years and thought I had a while left before it stopped working due to changes in Firefox's add-on infrastructure, but after updating to Firefox 52 it's no longer working for me. I can see the puzzle icons in their various places, but there are no actual toolbars.
This is a great disappointment (though I understand the developer's reasons for not maintaining this add-on). If anyone has workarounds to keep Puzzle Bars functioning for a while longer, or suggestions for other add-ons that provide similar functionality, I'd love to see them posted here. I just added Vertical Toolbar and it seems like a solid replacement for the lateral bar provided by Puzzle Bars: https://addons.mozilla.org/en-US/firefox/addon/vertical-toolbar/