Weird results with FF 59.0.1

[FrankConijn]

See http://prntscr.com/ixqi6s for the screenshot. It concerns Win 7 Pro, current on update maintenance. And FF has all settings to default, except the Refresh button, as per your instruction.

[jscher2000]

Firefox (or something) seems to be adding top and bottom margin to the left and right curves. This is better:

.tabbrowser-tab[selected="true"]::before {
  margin: 1px 0 0 -30px !important;
}
.tabbrowser-tab[selected="true"]::after {
  margin: 1px -30px 0 0 !important;
}
#main-window[uidensity="compact"] .tabbrowser-tab[selected="true"]::before {
  margin: -1px 0 0 -30px !important;
}
#main-window[uidensity="compact"] .tabbrowser-tab[selected="true"]::after {
  margin: -1px -30px 0 0 !important;
}
#main-window[uidensity="compact"] .tab-line[selected] {
  margin-top: -2px !important;
  opacity: 0.5 !important;
}

Definitely not perfect, but...