This theme is cool. I like it. But it can be better.
In my opinion, the sidebar is an element of the Firefox theme that should usually be tied to the color of the toolbar and not the color of the new tab page. Maybe even with a shade lower or higher than the color of the toobar itself. If the Sidebar you want to put dark is not a problem, that's fine, however look for a different shade from the color of the new tab page. When I look at the sidebar with all the folders closed, it looks fine but as soon as you open a couple of folders the drag bar appears and it doesn't look so good anymore.

Good Work.