Sexy Tabs. Take that, Curvy Tabs! #firefoxAustralis :P

userChrome.css

/* set default namespace to XUL */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

.tabbrowser-tab[selected] { margin-top: -1px !important; }

.tab-background-middle {
  border-left: none !important;
  border-right: none !important;
  margin: 0 -0.5px !important;
}

.tab-background,
.tabs-newtab-button {
  -moz-margin-end: 0 !important;
  -moz-margin-start: 0.5px !important;
}

.tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox {
  -moz-padding-end: 0 !important;
  -moz-padding-start: 0 !important;
}

.tab-background-start[selected=true]::after,
.tab-background-start[selected=true]::before,
.tab-background-start,
.tab-background-end,
.tab-background-end[selected=true]::after,
.tab-background-end[selected=true]::before {
  width: 1px !important;
}

.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]),
.tabs-newtab-button:hover {
  background-image: linear-gradient(rgba(255,255,255,0), rgba(255,255,255,1) 80%, rgba(255,255,255,1)) !important;
  background-size: 100% !important;
}

#tabbrowser-tabs[movingtab] > .tabbrowser-tab[afterselected]::before,
.tabbrowser-tab:not([selected]):not([afterselected-visible]):not([first-visible-tab])::before,
#tabbrowser-tabs[overflow] > .tabbrowser-tab[first-visible-tab]:not([selected])::before,
#tabbrowser-tabs:not([overflow]) > .tabbrowser-tab:not([selected])::after {
  -moz-margin-start: -1.5px !important;
  -moz-margin-end: -1.5px !important;
  background-image: url(chrome://browser/skin/tabbrowser/tab-separator.png) !important;
  background-size: 3px 100% !important;
  content: "" !important;
  display: -moz-box !important;
  margin-bottom: 1px !important;
  width: 3px !important;
}

.tabbrowser-tab[selected]::before,
.tabbrowser-tab[selected]::after {
  -moz-margin-start: -0.5px !important;
  -moz-margin-end: -0.5px !important;
  background-color: #aaa !important;
  background-size: 1px 100% !important;
  content: "" !important;
  display: -moz-box !important;
  margin-bottom: 1px !important;
  width: 1px !important;
}

.tabbrowser-tab[selected][first-visible-tab]::before {
  background-color: #eee !important;
  background-size: 1px 100% !important;
}

.tabs-newtab-button { width: 32px !important; }

Looks good. :)