title: Is there a FF equivalent to chrome.declarativeContent.onPageChanged?
body: <p>I'm porting a Chrome extension to Firefox WebExtensions and I'm stuck on finding a workaround for <code>chrome.declarativeContent.onPageChanged</code>.</p>
  
  <p>My FF Webextension contains a Page Action that must be shown when navigating on certain websites. However, none of the listeners in the available API seem to allow this.</p>
  
  <p>In particular I've tried:</p>
  
  <pre><code>chrome.runtime.onInstalled.addListener(onChange);
  chrome.tabs.onCreated.addListener(onChange);
  chrome.tabs.onActivated.addListener(onChange);
  chrome.tabs.onUpdated.addListener(onChange);
  chrome.webNavigation.onDOMContentLoaded(onChange);
  chrome.webNavigation.onCreatedNavigationTarget(onChange);
  </code></pre>
  
  <p>Are there any known workarounds?</p>