[This line](https://github.com/rails/web-console/blob/be84e6471c33bbbea17f6dfc9e5ed3c7c7c4bb57/lib/web_console/templates/console.js.erb#L430) is causing a deprecation warning in Firefox. The warning says "Use of Mutation Events is deprecated. Use MutationObserver instead." Reproduction steps: - Open a Rails app with web-console in Firefox (I use Nightly but it should be possible to trigger on any recent versions) - Cause an error (e.g. go to a route that doesn't exist) - Check the browser console, see the deprecation warning. Firefox version: `78.0a1 (2020-05-11) (64-bit)` Rails version: 6.0.3 Ruby version: 2.6.5 web-console version: 4.0.2