Rounded tabs for Firefox + Nord color theme

userChrome.css

/* Adapted from https://github.com/wilfredwee/photon-australis/blob/master/userChrome-dark.css */
/* Works for Firefox 89 and later */
/* Pair with the offical Nord color theme: https://addons.mozilla.org/en-US/firefox/addon/nord-theme */
/* This file should be located at ~/.mozilla/firefox/<profile-dir>/chrome/userChrome.css */
/* Simply restart Firefox and the css should load */

:root {
  --tab-curve-width: 30px;
  --tabs-border: transparent !important;
  --tab-border-radius: 0 !important;
}

.tabbrowser-arrowscrollbox .arrowscrollbox-scrollbox {
  padding-inline-start: 10px !important;
}

/* To be able to see the top border of the tab */
.tab-stack {
  margin-top: 2px !important;
}

/* To prevent tab label appearing on pinned tab in compact mode. */
.tab-content {
  padding: 0 12px !important;
}

/* When the window is maximized, the first pinned tab is properly displayed. */
#TabsToolbar  {
  padding-inline-start: 15px !important;
  min-height: 33px !important;
  max-height: 33px !important;
}

.tabbrowser-tab[first-visible-tab="true"] {
  padding-left: 12px !important;
}

#tabbrowser-tabs {
  margin-left: -15px !important;
}

.titlebar-placeholder {
  border: none !important;
}

.titlebar-spacer {
  border: none !important;
}

/* Styles to account for when tab scrollbar is visible, and margin-inline-start kicks in */
.tabbrowser-tab[pinned="true"] {
  width: 40px !important; /* Actual photon pinned tab is 41px, but 40px removes pixelated artifact from pinned tab favicon */
}

.tabbrowser-tab[pinned="true"][style^="margin-inline-start"] {
  padding-left: 12px !important;
}

/* Remove unneeded styles from Photon */
.tabbrowser-tab::before,
.tabbrowser-tab::after {
  border: none !important;
}

/* Remove uneeded styles from Proton */
.tabbrowser-tab {
  padding-inline: 0px !important;
}

.tab-background {
  margin-bottom: 0 !important;
  display: flex !important;
}

.tab-background > .tab-context-line {
  flex: 1 !important;
}

#tabbrowser-tabs[haspinnedtabs]:not([positionpinnedtabs]) > #tabbrowser-arrowscrollbox > .tabbrowser-tab[first-visible-unpinned-tab] {
  margin-inline-start: 0px !important;
}

/* Remove container colors */
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
  background-color: rgba(0,0,0,0) !important;
  height: inherit !important;
  border-radius: 0px !important;
  margin: 0px !important;
}

/* Windows 7 specific */
@media (-moz-os-version: windows-win7) {
  .tabbrowser-tab {
    background-color: transparent !important;
    border: none !important;
  }

  @media (-moz-windows-default-theme) {
    .tabbrowser-tab:not(:-moz-lwtheme) {
      background-color: transparent !important;
      border: none !important;
    }
  }
}

.tabbrowser-tab > .tab-stack > .tab-background {
  background-image: none !important;
  -moz-box-orient: horizontal !important;
  background-color: transparent !important;
  margin-top: 1px !important;
}

.tab-background[selected="true"] {
  border: none !important;
}

.tab-background {
  pointer-events: none !important;
}

.tab-line {
  display: none !important;
}

.tab-bottom-line {
  display: none !important;
}

/* Match height of new tab button (right svg) on hover */
#new-tab-button {
  margin: 0 !important;
}

/* overlap the tab curves */
.tab-background {
  -moz-margin-end: -15px !important;
  -moz-margin-start: -15px !important;
}

/* Remove blue animation on tab on page load complete */
.tab-loading-burst {
  display: none !important;
}

/* Begin tab background customizations */
.tab-background[selected="true"]::before {
  border: none !important;
  content: "" !important;
  width: 30px !important;
  min-height: 30px !important;
  display: -moz-box !important;
  background-repeat: no-repeat !important;
}

.tab-background[selected="true"]::after {
  border: none !important;
  content: "" !important;
  width: 30px !important;
  min-height: 30px !important;
  display: -moz-box !important;
  background-repeat: no-repeat !important;
}

.tab-background[selected="true"] > spacer {
  margin-top: 0px !important;
}

#tabs-newtab-button,
#new-tab-button {
  width: calc(36px + 30px) !important;
  margin-inline-start: -15px !important;
  margin-top: 1px !important;
}

/* Tab hover customizations */
.close-icon:hover {
  border-radius: 50% !important;
}

/* Regular tabs */
.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true])::before {
  display: -moz-box !important;
  background-repeat: no-repeat !important;
  content: "" !important;
  width: 30px !important;
  max-height: 30px !important;
  background-color: transparent !important;
}

.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true])::after {
  display: -moz-box !important;
  background-repeat: no-repeat !important;
  content: "" !important;
  width: 30px !important;
  max-height: 30px !important;
  background-color: transparent !important;
}

.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]) > spacer {
  margin-top: 0px !important;
}

#TabsToolbar[brighttext] > #tabbrowser-tabs > .tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected="true"]),
#tabs-newtab-button:hover,
#tabs-newtab-button:hover::before,
#tabs-newtab-button:hover::after {
  background-color: transparent !important;
}

/* New tab hover customizations */
.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected="true"]),
#tabs-newtab-button:hover {
  background-position: 0px 2px, 30px 4px , 36px 2px !important;
  background-repeat: no-repeat !important;
  background-size: 30px 30px, calc(100% - (2 * 30px)) 30px, 30px !important;
}

#tabs-newtab-button:hover > .toolbarbutton-icon {
  background: none !important;
  background-color: transparent !important;
}

/* Color specific customizations */
:root {
  --svg-selected-before: 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:rgb(59,66,82);background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>");

  --svg-selected-after: 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:rgb(59,66,82);background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>");

  --background-selected-middle:
    linear-gradient(rgba(0,0,0,0), rgba(0,0,0,0)),
    linear-gradient(
      transparent
      2px,
      rgb(59,66,82) 2px,
      rgb(59,66,82)
    ),
    none !important;

  --svg-hover-before: 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:rgba(255, 255, 255, .1);background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>");

  --svg-hover-after: 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:rgba(255, 255, 255,.1);background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>");

  --background-hover-middle:
    linear-gradient(rgba(0,0,0,0), rgba(0,0,0,0)),
    linear-gradient(
      transparent
      2px,
      rgba(255,255,255,.1) 2px,
      rgba(255,255,255,.1)
    ),
    none;

  --newtab-hover: 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='30px' 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:rgba(255,255,255,.1);background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>"),
    linear-gradient(rgba(255,255,255,.1), rgba(255,255,255,.1)),
    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:rgba(255,255,255,.1);background-repeat:no-repeat;height:100%;width:100%;' xmlns='http://www.w3.org/1999/xhtml'></div></foreignObject></svg>");
}


/* OS-specific color variables */
@media screen and (-moz-windows-theme) {
  :root {

  }
}

@media not screen and (-moz-windows-theme) {
  :root {

  }
}

/* Dark theme */
@media (-moz-windows-accent-color-in-titlebar) {
  :root[tabsintitlebar]:not(:-moz-window-inactive) {
    background-color: #000000 !important;
  }
}


.tab-background[selected="true"]::before {
  background-image: var(--svg-selected-before) !important;
}
.tab-background[selected="true"]::after {
  background-image: var(--svg-selected-after) !important;
}

.tab-background[selected="true"] > .tab-context-line {
  background-image: var(--background-selected-middle) !important;
}

.tabbrowser-tab[visuallyselected="true"] > .tab-stack > .tab-background {
  box-shadow: none !important;    
}


.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true])::before {
  background-image: var(--svg-hover-before) !important;
}

.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true])::after {
  background-image: var(--svg-hover-after) !important;
}

.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]) > .tab-context-line {
  background-image: var(--background-hover-middle) !important;
}

#tabs-newtab-button:hover {
  background-image: var(--newtab-hover) !important;
}

Just tested on FF version 89.0 and it does not work. (Displays a narrow curved tab next to selected tab)

I just updated it for Firefox 89! @Audio-Bill