So I did that. While the transparency is fixed, everything else is broken. Broken stuff so far: -Back button icon is missing. -Can't click on anything in the menu. -Entire customization screen is broken.
If you're using Firefox 56, I could see about whipping up a theme based on the latest CSS files. I'm not sure if that'll fix the issue, but it could be worth a shot.
I would recommend using K-Meleon or Pale Moon (which is what I'm using now) as previously mentioned. You'll have much better support in the long run, and more customization options than Chrome or Firefox Quantum.