There's a lot to like about this and it's pretty cool, but a few things would make it significantly more useful.
(1) Right now the back/forward buttons on your mouse can't work "inside" the sidebar, so quick navigation without hunting for links is difficult. (2) There doesn't appear to be any way of clicking links in the sidebar and having them open in the main window, like Open Link in Current Tab or something. (3) It doesn't respect default page zoom settings, which means on a 4K monitor the text can be really tiny. It does allow you to zoom within the sidebar, but it won't remember it if you close it and open again. (4) Lastly, and this is a big one, you can't drag it out to make it any wider than the default, so at least on high resolution monitors, it's a just a thin little box on the side. Imagine if you could drag it out to the 50% mark and easily navigate within it -- you could essentially have dual pane tabs! Which would be amazing, and huge for all sorts of workflows.
Anyway, just to be clear, I do like this addon quite a bit, but with some small tweaking it could be so much more!