Tab groups.

[codecnotsupported]

There isn't a underline when marking tab group

I commented out:

firefox-sweet-theme/theme/parts/tabsbar.css

Lines 109 to 119 in d5fdc33

	.tabbrowser-tab {
	border-width: 0 !important;
	/* padding: 5px 2px 6px !important; */
	position: relative;
	}
	.tabbrowser-tab:not([hidden=true], [pinned]):first-of-type {
	padding-left: 5px !important;
	}
	.tabbrowser-tab:not([hidden="true"]):last-of-type {
	padding-right: 5px !important;
	}

And to fix the tab groups not contracting I commented out:

firefox-sweet-theme/theme/parts/tabsbar.css

Lines 402 to 410 in d5fdc33

	.tabbrowser-tab:not([style^="max-width"]):not([pinned]),
	.tabbrowser-tab[style^="max-width: 100px !important;"]:not([pinned]) {
	max-width: 100% !important;
	}
	.tabbrowser-tab:not([style^="max-width"]):not([pinned]):not([fadein]),
	.tabbrowser-tab[style^="max-width: 100px !important;"]:not([pinned]):not([fadein]) {
	max-width: .1px !important;
	}

Which worked.
May I just add that it is unreasonably effective to simply remove parts of CSS until you see what causes the breakage.