This is a neat idea, and it shows off how powerful the "limited" WebExtension API really is. Nicely done!

If you're comfortable with editing your userChrome.css file, it's possible to have this effect applied to your Bookmarks and History sidebars as well. Details here: https://www.reddit.com/r/FirefoxCSS/comments/7wmewg/i_know_its_a_long_shot_but_can_we_apply/du2cfrb/

I don't think it's possible to apply to other sidebars, but I'm not the expert.