Target Mozilla Thunderbird using .moz-text-html CSS Class

You can target the Mozilla Thunderbird email client by including the CSS class .moz-text-html, which Thunderbird wraps around the contained email.

  <style type="text/css">
    .moz-text-html { 
        {Place your styles here} 
    }
  </style>

Additionally, as advanced CSS within the -webkit-device-pixel-ratio media queries does not work, you can use -moz-device-pixel-ratio to set the styles that trigger in Thunderbird only.

  <style type="text/css">
    @media screen and (-moz-device-pixel-ratio){
     .span{ color: #cc0000; }
    }
  </style>