Install dark reader extension

Dark Reader

Update the userChrome.css and userContent.css

@-moz-document url(chrome://browser/content/browser.xhtml) {
  #main-window,
  browser[type="content-primary"],
  browser[type="content"],
  tabbrowser#content,
  #content,
  browser[type="content"] > html {
    background: #323234 !important;
  }
}
  • userContent.css
@charset "utf-8"; /* CSS Document */

@-moz-document url("about:newtab") {
  body {
    background-color: #011326 !important;
  }
}

@-moz-document url(chrome://browser/content/browser.xhtml) {
  browser[type="content-primary"] {
    background: #011326 !important;
  }
}

Update firefox flags

toolkit.legacyUserProfileCustomizations.stylesheets: true
browser.startup.blankWindow: false

Set dark theme in firefox browser

  • Use any dark theme
  • Set the Option in firefox setting: Override the colors specified by the page with your selections above: Never

Themes shortcut

ms-settings:themes