I was testing macOS High Sierra Beta, and I used Firefox Nightley 58 until yesterday. The close/minimize/maximize buttons dissapeared on the light and dark theme, while default showed an imperfect black round corner and green decoloration.
Today I upgrade to High Sierra final release, and change browser to Firefox Quantum, 57.b03. The problem persists. I didn’t have the same issue in Firefox Nightley 58 in Windows 10.