Before and after: https://i.imgur.com/LsuJFZA.png

userChrome.css

.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
	display: none;
}

.tabbrowser-tab[class*="identity-color-"] .tab-background {
	background: linear-gradient(var(--identity-tab-color) 2px, transparent 0);
}

.tabbrowser-tab[selected="true"][class*="identity-color-"] .tab-background {
	background: linear-gradient(var(--identity-tab-color) 2px, var(--lwt-selected-tab-background-color) 0) !important;
}