You can use this addon: https://addons.mozilla.org/de/firefox/addon/foxreplace/?src=...

Apply an autoload rule with these settings:

  Name: zero length replacement
  HTML: Output only
  URL: <entering no url matches all sites>
  Replace: (\uFEFF|\u200B|\u200C)
  Type: Regular expression
  With: <span style="background-color:#F00 !important">&#x1f633;</span>