title: How to check if a Firefox WebExtension is installed or not with page JavaScript?
body: <p>I have developed a WebExtension for Firefox and my website works with the extension as a prerequisite. I need to check programmatically whether the extension is installed or not and if not ask the user to install it.</p>
  
  <p>I am not able to find a way how to check this operation of whether my extension is already installed in the user's browser.</p>
  
  <p>Editor note: Methods available in Firefox differ from those <a href="https://stackoverflow.com/questions/6293498/check-whether-user-has-a-chrome-extension-installed">available in Chrome</a>, so this question is not a duplicate.</p>