How to increase the contrast of Firefox tabs
Apply this CSS somehow, for example through stylish:
.tabbrowser-tab .tab-label {     opacity: 1 !important;
  color: black !important;
  background-color: white; }
.tabbrowser-tab[pending], .alltabs-item[pending] {
    opacity: 1 !important;
  color: black !important;
  background-color: white;
 
}
If you use Stylish add this namespace line first:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
I used Firefox - style of inactive or unloaded tabs - Themes and Skins for Browser - userstyles.org as a template.