Steps to reproduce:

1) Disable all add-ons in Firefox
2) Open an URL that points to a XML file
3) AS EXPECTED: A pretty-printed XML is shown
4) Enable the React DevTools Add-on
5) Open the XML file again
6) FAIL: Only the content inside the XML tags are shown

Using Firefox Developer Edition 72.0b6 on macOS 10.15.2
import nullthrows from 'nullthrows';
import {installHook} from 'react-devtools-shared/src/hook';
import {SESSION_STORAGE_RELOAD_AND_PROFILE_KEY} from 'react-devtools-shared/src/constants';
import {sessionStorageGetItem} from 'react-devtools-shared/src/storage';
function injectCode(code) {
  const script = document.createElement('script');
  script.textContent = code;
  
  
  nullthrows(document.documentElement).appendChild(script);