Ill put in a ticket with Mozilla to bring back Firefox 1.0 UI for you.
[Edit].
Sorry bro, i guess support.mozilla.org blocks VPNs, so I cant file a ticket. Unfortunate. But just know I really wanted to get back the contrastiest contrast and sharpest corners possible for you. Hopefully one day Mozilla lifts the VPN ban so I can put that ticket in.
Good luck with the rounder corners and custom color themes in the meantime. ❤