Firefox EventTarget bug reproduction

content.js

console.log('Test 1: raw EventTarget');
const et = new EventTarget();                                               
// add handler of 'foo' event
et.addEventListener('foo', (e)=> {                                          
    console.log('foo event emitted:', e.detail);                            
});
// dispatch 'foo' event                                                     
et.dispatchEvent(new CustomEvent('foo', { detail: 123 }));                  
      
console.log('Test 2: subclass of EventTarget');                             
class MyEventTarget extends EventTarget {}                                  
const met = new MyEventTarget();                                            
// add handler of 'foo' event
met.addEventListener('foo', (e)=> {
    console.log('foo event emitted:', e.detail);
});
// dispatch 'foo' event
met.dispatchEvent(new CustomEvent('foo', { detail: 456 }));

manifest.json

{                                      
  "manifest_version": 2,                                                    
  "name": "Firefox EventTarget bug reproduction",                           
  "version": "1.0.0",                                                       
  "description": "sample",                                                  
  "content_scripts": [                                                      
    {                                                                       
      "matches": ["<all_urls>"],                                            
      "js": ["content.js"]                                                  
    }                                                                       
  ]                                                                         
}