I am generally not a fan of theming an app to look like another app/environment/whatever, BUT I am also tired of Firefox on GNOME feeling stuck five years ago visually...
So I am giving https://github.com/rafaelmardojai/firefox-gnome-theme a shot. It's... interesting! I like that it does really look like GNOME Web at a glance and feels more at home in Adwaita, but at the same time, there are clearly issues here and there because it tries to theme EVERYTHING.