Customize Firefox Proton Tabs

userChrome.css

/*
  @see https://www.reddit.com/r/FirefoxCSS/comments/73dvty/tutorial_how_to_create_and_livedebug_userchromecss/
  @see https://www.reddit.com/r/firefox/comments/mxqc71/this_community_complains_a_lot_but_this_time_i/
  @see https://gist.github.com/Speedy37/63d37931601123178ac2221533292cdd
*/
.tab-stack {
    margin-top: 1px;
}
.tabbrowser-tab {
    margin: 0px !important;
    margin-inline: 0px !important;
    padding-inline: 0px !important;
}
.tab-background {
    margin: 0px !important;
    border-bottom-left-radius: 0 !important;
    border-bottom-right-radius: 0 !important;
}
.tab-background[selected="true"] {
    background: linear-gradient(to bottom, rgb(64, 70, 168) 2px, var(--toolbar-bgcolor) 2px, var(--toolbar-bgcolor) 100%) !important;
}
.tabbrowser-tab:not([selected="true"]) + .tabbrowser-tab:not([selected="true"]) {
    border-radius: 0 !important;
    border-left: 1px solid var(--tabs-border-color) !important;
}
.tabbrowser-tab:not([selected="true"]) + #tabs-newtab-button {
    border-radius: 0 !important;
    border-left: 1px solid var(--tabs-border-color) !important;
}
.tab-context-line {
    margin-top: 26px !important;
    margin-left: 6px !important;
    margin-right: 6px !important;
}
menupopup > menuitem, menupopup > menu {
    padding-block: 0.15em !important;
}
.panel-subview-body > * {
    padding: 2px 6px !important;
}