Firefox userChrome.css

userChrome.css

@media (-moz-proton) {
  #tabbrowser-tabs {
    --proton-tab-block-margin: 0;
    --tab-border-radius: 0;
  }

  .tab-background {
    box-shadow: none !important;
    margin: 0 !important;
  }

  #tabbrowser-tabs .tab-background > .tab-context-line {
    margin: 0 !important;
  }

  .tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
    height: 1px !important;
  }
  
  .tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line + .tab-loading-burst.proton {
    background-color: var(--identity-icon-color);
    opacity: 0.1;
  }
}

I use the Containers extension, and added these rules to tint containered tabs with the accent color:

  .tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
    height: 1px !important;
  }
  
  .tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line + .tab-loading-burst {
    background-color: var(--identity-tab-color);
    opacity: 0.1;
  }

It looks like this:

Edit: Firefox 91 made some changes, remove the .proton in the last rule, and the proton in the variable on line 3 to make it work again.

Nice - gonna use that as well. Thanks for sharing!

Thank you! Do you know if it's possible to also get rid of this gap between tabs completely?

Edit: and the padding around the address bar?
Edit 2: maybe it's the close button