Firefox-UI-Fix/src/tab/newtab_button/_looks_like_tab.scss

#tabs-newtab-button {
  @include Option("userChrome.tab.connect_to_window") {
    /* Size */
    -moz-box-align: stretch !important;
    padding-top: var(--tab-block-margin) !important;
  }

  @include Option("userChrome.tab.bottom_rounded_corner") {
    /* Original:
    margin: 0 0 var(--tabs-navbar-shadow-size) !important
    => Can't override style. Therefore, we should approach it by bypass.
   */
    --tabs-navbar-shadow-size: -1px; /* Original: 1px */
    --uc-tabs-navbar-shadow-size: 0.5px;
    --uc-tab-corner-rounding: 4px; /* Hardcoded */
    --uc-newtab-bgcolor: transparent;

    /* Corner Rounding Image */
    --uc-newtab-position: calc((var(--uc-tab-corner-rounding) / 2) * -1);
    background-image: var(--uc-tab-corner-left-side-svg), var(--uc-tab-corner-right-side-svg);
    background-repeat: no-repeat;
    background-position: left var(--uc-newtab-position) bottom var(--uc-tabs-navbar-shadow-size),
                         right var(--uc-newtab-position) bottom var(--uc-tabs-navbar-shadow-size);
    background-size: var(--uc-tab-corner-rounding);

    /* Corner Rounding Color */
    fill: var(--uc-newtab-bgcolor) !important;
    -moz-context-properties: fill !important;

    @include Option(
      "userChrome.tab.bottom_rounded_corner.wave",
      "userChrome.tab.bottom_rounded_corner.australis",
      "userChrome.tab.bottom_rounded_corner.chrome_legacy",
      "userChrome.tab.bottom_rounded_corner.chrome",
      "userChrome.tab.bottom_rounded_corner.edge"
    ) {
      --uc-newtab-position: 0px;
      --uc-newtab-non-corner-bgwidth: 0px;
      --uc-tab-corner-half-height: calc(var(--uc-tab-corner-height) / 2);
      --uc-tab-corner-half-height-reverse: calc(var(--uc-tab-corner-half-height) * -1);

      transform: translateX(var(--uc-tab-corner-half-height-reverse));
      padding-inline: var(--uc-tab-corner-half-height) !important;
      overflow-clip-margin: var(--uc-tab-corner-half-height) !important;

      background-image: var(--uc-tab-corner-left-side-svg),
                        linear-gradient(to left, var(--uc-newtab-bgcolor), var(--uc-newtab-bgcolor)),
                        var(--uc-tab-corner-right-side-svg);
      background-position: left var(--uc-newtab-position) bottom var(--uc-tabs-navbar-shadow-size),
                           center bottom var(--uc-tabs-navbar-shadow-size),
                           right var(--uc-newtab-position) bottom var(--uc-tabs-navbar-shadow-size);
      background-size: var(--uc-tab-corner-height) var(--uc-tab-corner-height),
                       var(--uc-newtab-non-corner-bgwidth) var(--uc-tab-corner-height),
                       var(--uc-tab-corner-height) var(--uc-tab-corner-height);
      background-origin: padding-box;

      @include Option(
        "userChrome.tab.bottom_rounded_corner.chrome",
        "userChrome.tab.bottom_rounded_corner.edge"
      ) {
        --uc-newtab-non-corner-bgwidth: calc(2 * var(--toolbarbutton-inner-padding));
      }
    }
  }
}

/* Corner Rounding Color */
#tabs-newtab-button:hover {
  --uc-newtab-bgcolor: var(--toolbarbutton-hover-background);
}
#tabs-newtab-button:hover:active {
  --uc-newtab-bgcolor: var(--toolbarbutton-active-background);
}
@media (-moz-windows-accent-color-in-titlebar) {
  @include AccentColor {
    :root[tabsintitlebar]:not(:-moz-window-inactive, :-moz-lwtheme) #tabs-newtab-button:hover,
    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
      fill: color-mix(
        in srgb,
        $accentTextColor 10%,
        transparent
      ) !important; /* Hardcoded for compatibility */
    }
    :root[tabsintitlebar]:not(:-moz-window-inactive, :-moz-lwtheme) #tabs-newtab-button:hover:active,
    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
      fill: color-mix(
        in srgb,
        $accentTextColor 15%,
        transparent
      ) !important; /* Hardcoded for compatibility */
    }

    @include Option("userChrome.tab.bottom_rounded_corner") {
      @include Option(
        "userChrome.tab.bottom_rounded_corner.wave",
        "userChrome.tab.bottom_rounded_corner.australis",
        "userChrome.tab.bottom_rounded_corner.chrome_legacy",
        "userChrome.tab.bottom_rounded_corner.chrome",
        "userChrome.tab.bottom_rounded_corner.edge"
      ) {
        :root[tabsintitlebar]:not(:-moz-window-inactive, :-moz-lwtheme) #tabs-newtab-button:hover,
        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
          background-image: var(--uc-tab-corner-left-side-svg),
                            linear-gradient(to left, color-mix(in srgb, $accentTextColor 10%, transparent), color-mix(in srgb, $accentTextColor 10%, transparent)),
                            var(--uc-tab-corner-right-side-svg);
        }
        :root[tabsintitlebar]:not(:-moz-window-inactive, :-moz-lwtheme) #tabs-newtab-button:hover:active,
        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
          background-image: var(--uc-tab-corner-left-side-svg),
                            linear-gradient(to left, color-mix(in srgb, $accentTextColor 15%, transparent), color-mix(in srgb, $accentTextColor 15%, transparent)),
                            var(--uc-tab-corner-right-side-svg);
        }
      }
    }
  }
}

/* '+'Icon */
#tabs-newtab-button .toolbarbutton-icon {
  border-radius: var(--tab-border-radius) var(--tab-border-radius) 0 0 !important; /* Original: var(--tab-border-radius) */

  padding: calc(var(--toolbarbutton-inner-padding) - (var(--tab-block-margin) / 4)) var(--toolbarbutton-inner-padding)
    calc(var(--toolbarbutton-inner-padding) + (var(--tab-block-margin) / 4) + var(--uc-tabs-navbar-shadow-size)) !important;
  -moz-context-properties: fill, fill-opacity;
  fill: var(--toolbarbutton-icon-fill);
  fill-opacity: var(--toolbarbutton-icon-fill-opacity);

  @include Option(
    "userChrome.tab.bottom_rounded_corner.wave",
    "userChrome.tab.bottom_rounded_corner.australis",
    "userChrome.tab.bottom_rounded_corner.chrome_legacy",
    "userChrome.tab.bottom_rounded_corner.chrome",
    "userChrome.tab.bottom_rounded_corner.edge"
  ) {
    --tab-border-radius: 0px;
    width: unset !important;
    padding-inline: 0px !important;
    margin-inline: calc(var(--uc-tab-corner-half-height) - #{math.div(15.5px, 2)}) !important;

    background-color: transparent !important;
    // background-size: calc((var(--toolbarbutton-inner-padding) * 2 + 16px) - var(--uc-tab-corner-size)) 100%;
    // background-repeat: no-repeat;
    // background-position: center;
  }
}