Кажется, что звездочки 4.0.0 совместимы с Chrome 78, но не с Firefox 70.
Начиная с нового приложения rails:
rails -v # Rails 6.0.0
rails new helloworld
# Gemfile
gem 'sass-rails', '~> 6' # not 5
bundle | grep sprockets
Using sprockets 4.0.0
Using sprockets-rails 3.2.1
# layouts/application.html.erb
javascript_include_tag 'application'
# app/assets/javascripts/application.js
console.log('line 1');
throw 'line 2';
# app/assets/config/manifest.js
//= link_directory ../javascripts .js
# Dev tools console, Firefox 70
line 1 application.source.js:1
uncaught exception: line 2 application.source.js:2
Error while fetching an original source: "http://localhost:3000/assets/application.source.js" is not in the SourceMap.
Source URL: <unknown>
Прекрасно работает в Chrome 78