I really like how simple this add-on is. The icon looks like an inverted "show history" icon in the address bar. It looks organic in Firefox UI. Also I like the idea that the icon is on toolbar. BUT like Albert71292 said, the extension needs to automatic switching to a lighter color when the theme is dark. Also the extension color is a bit darker than the "show history" icon. I am a perfectionist, so it annoys me a little :)
Please, pay attention to my review! I spent a lot of time writing this review, since I don’t know english well. I will appreciate that.