fenix-fox/chrome/non_floating_sharp_tabs.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/non_floating_sharp_tabs.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* This style makes tabs and related items non-rounded and connects tabs to toolbars like in previous Firefox versions.
 * You should set layout.css.color-mix.enabled to true in about:config to make colors apply properly.
 * It's not strictly a requirement, but this style expects compact_proton.css to be loaded before it. */

:root[id]{
  --tab-block-margin: 0px !important;
  --tabs-shadow-size: 1px !important;
  
  /* Remove next line if you want selected tab to have color other than toolbar background - then it follows your theme color  */
  --lwt-selected-tab-background-color: var(--toolbar-bgcolor) !important; 
}

/* Uncomment next line to force specific color for tab top line */
/* #tabbrowser-tabs{ --tab-line-color: blue !important; } */

/* These next two rules set a color for border around tabs and between tabs & navigation toolbars. Set to transparent to remove the border.*/
#nav-bar{
  /* This overrides value in compact_proton.css */
  box-shadow: 0 -1px 0 0 color-mix(in srgb, currentcolor 30%, transparent) !important;
}
.tab-background[selected]{
  border-inline: 1px solid color-mix(in srgb, currentcolor 30%, transparent) !important;
}

#TabsToolbar{
  --toolbarbutton-inner-padding: 7px !important;
}
/* Few exceptions for default light theme */
#navigator-toolbox:not([movingtab]):-moz-lwtheme > #titlebar > #TabsToolbar{
  --toolbar-bgcolor: transparent;
}
#TabsToolbar:not(:-moz-lwtheme){ --tab-line-color: transparent; }
.tab-background[selected]:not(:-moz-lwtheme){ background: var(--toolbar-bgcolor) !important; }

#TabsToolbar .toolbarbutton-1 > .toolbarbutton-badge-stack,
#TabsToolbar .toolbarbutton-1 > .toolbarbutton-icon{ border-radius: 2px !important;  }

/* tabs newtab button needs some special styling... */
#tabs-newtab-button{ padding-inline: 0 !important; }
#tabbrowser-arrowscrollbox > #tabs-newtab-button > .toolbarbutton-icon{
  border-radius: 0 !important; 
  width: initial !important; 
  height: initial !important;
  padding: 9px !important;
}
:root[uidensity="compact"] #tabbrowser-arrowscrollbox > #tabs-newtab-button > .toolbarbutton-icon{
  padding: 7px !important;
}

#scrollbutton-up,
#scrollbutton-down{ border-radius: 0 !important; border-width: 0 !important; padding-inline: 3px !important; }

/* tab shaping */
.tabbrowser-tab{ padding-inline: 0 !important; }

:root[uidensity="compact"] #tabbrowser-tabs[positionpinnedtabs] .tabbrowser-tab[pinned]{ min-height: calc(var(--tab-min-height) + 2px) !important; }

.tab-content[pinned]{ padding-inline: 11px !important; }

.tab-background{
  border-radius: 0 !important;
  box-shadow: none !important;
  border-top: 0 !important;
}

.tab-background:not([selected])[multiselected]{
  background: color-mix(in srgb, currentColor 11%, transparent) !important;
  margin-inline-start: -1px; 
}

/* Line to mark selected tab */
.tab-background[selected]::before,
.tabbrowser-tab:hover > stack > .tab-background::before{
  display: -moz-box;
  height: 2px;
  content: "";
}
.tab-stack:hover > .tab-background::before{
  background-color: inherit;
}
.tab-stack > .tab-background[selected]::before{
  background-color: highlight;
  background-image: linear-gradient(var(--tab-line-color),var(--tab-line-color));
}
/* Photon-like tab on hover animation for the top line */
@keyframes tab-onhover-line-anim{ from{ margin-inline: 20px } to { margin-inline: 0 } }

.tab-stack:hover > .tab-background::before{ animation: tab-onhover-line-anim 160ms }

/* Disable animation for selected and pinned tabs */
.tabbrowser-tab:is([pinned],[selected]) > .tab-stack > .tab-background::before{ animation: none }

/* moves context-line to the bottom */
.tab-context-line{ -moz-box-ordinal-group: 2; margin-inline: 10px !important; }