Deprecation warning for web console in Firefox

[connorshea]

This line 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