A browser chosen on 3.x stays through the Quattro upgrade
A change merged into quattro sets Chromium as the default browser only when no default resolves, so a browser chosen on 3.x is kept through the Quattro upgrade.
The Quattro upgrade reset every user's default browser to Chromium. Near the end of the user transition it set Chromium unconditionally. On 3.x, choosing Firefox rewrites the http, https, and text/html handlers, so the file that holds them is kept, and then the upgrade overwrote the choice.
Chromium is set only when the current default resolves to nothing. An untouched install still lands on Chromium through the package-owned list. A user file that names a browser whose desktop file is missing still resolves to Chromium.
The terminal half of the same issue is left for the maintainer. Files written for Ghostty or Alacritty match the stock files, so the retire step cannot tell a choice from an untouched default.