zifiridark is a OLED-friendly theme for Mozilla Firefox and Mozilla Thunderbird.
I highly recommend using these userChrome.css settings along with zifiridark:
.tab-background {
margin-top: 0px !important;
margin-bottom: 0px !important;
border-radius: 0px 0px !important;
/*Change currentColor value to tweak background tab shade because shading may vary depending your display.*/
background-color: color-mix(in srgb, currentColor 13%, transparent);
}
.tabbrowser-tab[selected=true] .tab-background ,
.tabbrowser-tab[multiselected=true] .tab-background {
background-color: var(--toolbar-bgcolor) !important;
background-image: var(--toolbar-bgimage) !important;
}
/*Shortens tabs vertically, more space for webpages.*/
.tabbrowser-tab {
max-height: var(--tab-min-height) !important;
}