Australis-like tabs for Lepton (https://github.com/black7375/Firefox-UI-Fix / https://waterfox.net/ default theme)

australis-for-lepton.userChrome.css

.tabbrowser-tab {
  --tab-corner-rounding: 30px;
  --toolbar-color: transparent;

  overflow: visible !important;
}

/* Fix left curve display on first tab */
#tabbrowser-arrowscrollbox:not([overflowing="true"]) .titlebar-spacer {
  margin-inline-end: -15px;
}
#tabbrowser-arrowscrollbox:not([overflowing="true"]) .tabbrowser-tab:first-child::before {
  content: "";
  display: inline-block;
  width: 15px;
}

.tab-context-line {
  opacity: 0 !important;
  visibility: hidden;
}

/* Roundings shouldn't capture hover */
.tabbrowser-tab > stack::before,
.tabbrowser-tab > stack::after {
  pointer-events: none;
}

.tabbrowser-tab[visuallyselected] > stack::before {
  left: -15px !important;
  background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' xmlns:svg='http://www.w3.org/2000/svg' width='30px' height='31px' preserveAspectRatio='none'><defs><svg:clipPath id='tab-curve-clip-path-start' clipPathUnits='objectBoundingBox'><svg:path d='m 1,0.065 0.05,0 0,0.938 -1,0 0,-0.028 C 0.32082458,0.95840561 0.4353096,0.81970962 0.48499998,0.5625 0.51819998,0.3905 0.535,0.0659 1,0.065 z'/></svg:clipPath><svg:clipPath id='tab-curve-clip-path-end' clipPathUnits='objectBoundingBox'><svg:path d='m 0,0.065 -0.05,0 0,0.938 1,0 0,-0.028 C 0.67917542,0.95840561 0.56569036,0.81970962 0.51599998,0.5625 0.48279998,0.3905 0.465,0.0659 0,0.065 z'/></svg:clipPath><svg:clipPath id='tab-hover-clip-path' clipPathUnits='objectBoundingBox'><svg:path d='M 0,0.2 0,1 1,1, 1,0.2 z'/></svg:clipPath></defs><foreignObject width='30' height='31' clip-path='url(%23tab-curve-clip-path-start)'><div id='tab-background-fill' style='background-color:%232b2a33;background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>") !important;
}
.tabbrowser-tab[visuallyselected] > stack::after {
  right: -15px !important;
  background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' xmlns:svg='http://www.w3.org/2000/svg' width='30px' height='31px' preserveAspectRatio='none'><defs><svg:clipPath id='tab-curve-clip-path-start' clipPathUnits='objectBoundingBox'><svg:path d='m 1,0.065 0.05,0 0,0.938 -1,0 0,-0.028 C 0.32082458,0.95840561 0.4353096,0.81970962 0.48499998,0.5625 0.51819998,0.3905 0.535,0.0659 1,0.065 z'/></svg:clipPath><svg:clipPath id='tab-curve-clip-path-end' clipPathUnits='objectBoundingBox'><svg:path d='m 0,0.065 -0.05,0 0,0.938 1,0 0,-0.028 C 0.67917542,0.95840561 0.56569036,0.81970962 0.51599998,0.5625 0.48279998,0.3905 0.465,0.0659 0,0.065 z'/></svg:clipPath><svg:clipPath id='tab-hover-clip-path' clipPathUnits='objectBoundingBox'><svg:path d='M 0,0.2 0,1 1,1, 1,0.2 z'/></svg:clipPath></defs><foreignObject width='30' height='31' clip-path='url(%23tab-curve-clip-path-end)'><div id='tab-background-fill' style='background-color:%232b2a33;background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>") !important;
}

.tabbrowser-tab:hover:not([visuallyselected]) > stack::before,
.tabbrowser-tab:hover:not([visuallyselected]) > stack::after {
  content: "" !important;
  display: block !important;
  position: absolute !important;
  z-index: 1 !important;
  width: var(--tab-corner-rounding) !important;
  height: 100% !important;
  background-size: var(--tab-corner-rounding);
  background-repeat: no-repeat;
  background-position-y: bottom;
}

.tabbrowser-tab:hover:not([visuallyselected]) > stack::before {
  left: -15px !important;
  background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' xmlns:svg='http://www.w3.org/2000/svg' width='30px' height='31px' preserveAspectRatio='none'><defs><svg:clipPath id='tab-curve-clip-path-start' clipPathUnits='objectBoundingBox'><svg:path d='m 1,0.065 0.05,0 0,0.938 -1,0 0,-0.028 C 0.32082458,0.95840561 0.4353096,0.81970962 0.48499998,0.5625 0.51819998,0.3905 0.535,0.0659 1,0.065 z'/></svg:clipPath><svg:clipPath id='tab-curve-clip-path-end' clipPathUnits='objectBoundingBox'><svg:path d='m 0,0.065 -0.05,0 0,0.938 1,0 0,-0.028 C 0.67917542,0.95840561 0.56569036,0.81970962 0.51599998,0.5625 0.48279998,0.3905 0.465,0.0659 0,0.065 z'/></svg:clipPath><svg:clipPath id='tab-hover-clip-path' clipPathUnits='objectBoundingBox'><svg:path d='M 0,0.2 0,1 1,1, 1,0.2 z'/></svg:clipPath></defs><foreignObject width='30' height='31' clip-path='url(%23tab-curve-clip-path-start)'><div id='tab-background-fill' style='background-color:%2335343a;background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>") !important;
}
.tabbrowser-tab:hover:not([visuallyselected]) > stack::after {
  right: -15px !important;
  background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' xmlns:svg='http://www.w3.org/2000/svg' width='30px' height='31px' preserveAspectRatio='none'><defs><svg:clipPath id='tab-curve-clip-path-start' clipPathUnits='objectBoundingBox'><svg:path d='m 1,0.065 0.05,0 0,0.938 -1,0 0,-0.028 C 0.32082458,0.95840561 0.4353096,0.81970962 0.48499998,0.5625 0.51819998,0.3905 0.535,0.0659 1,0.065 z'/></svg:clipPath><svg:clipPath id='tab-curve-clip-path-end' clipPathUnits='objectBoundingBox'><svg:path d='m 0,0.065 -0.05,0 0,0.938 1,0 0,-0.028 C 0.67917542,0.95840561 0.56569036,0.81970962 0.51599998,0.5625 0.48279998,0.3905 0.465,0.0659 0,0.065 z'/></svg:clipPath><svg:clipPath id='tab-hover-clip-path' clipPathUnits='objectBoundingBox'><svg:path d='M 0,0.2 0,1 1,1, 1,0.2 z'/></svg:clipPath></defs><foreignObject width='30' height='31' clip-path='url(%23tab-curve-clip-path-end)'><div id='tab-background-fill' style='background-color:%2335343a;background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>") !important;
}

/* Show content above the curves */
.tabbrowser-tab:hover .tab-content,
.tabbrowser-tab[visuallyselected] .tab-content {
  z-index: 2;
}

/* Active tab background shouldn't overlap the curves */
.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected="true"], [multiselected]),
.tab-background[selected="true"]:-moz-lwtheme {
  background-size: calc(100% - 30px) 29px !important;
  background-repeat: no-repeat !important;
  background-position: 15px bottom !important;
}
.tab-background[selected="true"]:-moz-lwtheme {
  background-image: linear-gradient(#2b2a33, #2b2a33), none, none !important;
}
.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected="true"], [multiselected]) {
  background-color: transparent !important;
  background-image: linear-gradient(#35343a, #35343a), none, none !important;
}

/* Shorter separators */
.tab-background::after {
  height: 50%;
}

/* Don't display separator before the tabs */
.tab-background::before {
  display: none;
}

/* Hide separators around hovered tab */
.tabbrowser-tab[beforehovered="true"] > .tab-stack > .tab-background::after,
.tabbrowser-tab:hover > .tab-stack > .tab-background::after {
  opacity: 0;
}