CSS code to get Firefox' Australis theme usable (features: tabs on bottom; rectangular tabs; small space saving back/forward button; tweaked colors in tabbar)

userChrome.css

/* FIXES TO AUSTRALIS THEME */
/* bookmarks toolbar below adress bar */
#PersonalToolbar {
  -moz-box-ordinal-group: 2 !important;
}
/* tabs bar below adress bar and bookmarks toolbar */
#TabsToolbar {
  -moz-box-ordinal-group: 3 !important;
  margin: 0 !important;
  padding: 0 !important;
  background: transparent !important;
}
/* tweak menubar design */
#toolbar-menubar {
  margin: 0 !important;
  margin-top: 2px !important;
  padding: 0 !important;
  padding-left: 16px !important;
  background: url("chrome://branding/content/icon16.png") 0 50% no-repeat !important;
  text-shadow: 0 0 0.8em #FFF, 0 0 0.8em #FFF, 0 0 0.5em #FFF, 0 1px 0 rgba(255, 255, 255, 0.5) !important;
}
/* tweak nav-bar design */
#nav-bar {
  margin: 0 !important;
  padding: 0 !important;
  background: none !important;
  border: none !important;
  box-shadow: none !important;
}
#nav-bar .toolbarbutton-1:not([type=menu-button]),
#nav-bar .toolbarbutton-1 > .toolbarbutton-menubutton-button,
#nav-bar .toolbarbutton-1 > .toolbarbutton-menubutton-dropmarker {
  padding: 0 !important;
}
/* show overflow (notably the small arrow when inserting a new tab) */
#navigator-toolbox {
  overflow: visible !important;
}
/* shift windows buttons a bit to prevent overlap when Aero is turned off */
@media not all and (-moz-windows-compositor) {
  #main-window[sizemode="normal"] #titlebar-buttonbox {
    margin-top: -7px;
  }
}


/* de-fog tabbar and invert buttons for better visibility */
#TabsToolbar:not(:-moz-lwtheme)::after {
  margin: 0 30px !important;
  box-shadow: 0 10px 20px 20px #fff !important;
  opacity: 0.5 !important;
}
#TabsToolbar .tabs-newtab-button,
#TabsToolbar #new-tab-button > .toolbarbutton-icon {
  list-style-image: url("chrome://browser/skin/tabbrowser/newtab-inverted.svg") !important;
}
#TabsToolbar #alltabs-button[type="menu"],
#TabsToolbar .toolbarbutton-1 .dropmarker-icon {
  list-style-image: url("chrome://browser/skin/toolbarbutton-dropdown-arrow-inverted.png") !important;
}


/* rectangular space-saving tabs (partially from http://userstyles.org/styles/99560/square-tab-for-firefox-australis) */
#TabsToolbar .arrowscrollbox-scrollbox {
  padding: 0 !important;
}
#TabsToolbar .tabbrowser-tabs {
  min-height: 26px !important;
}
#TabsToolbar .tabbrowser-tab {
  -moz-border-top-colors: none !important;
  -moz-border-left-colors: none !important;
  -moz-border-right-colors: none !important;
  -moz-border-bottom-colors: none !important;
  border-style: solid !important;
  border-color: rgba(0,0,0,.2) !important;
  border-width: 1px 1px 0 !important;
  border-radius: 2px 2px 0 0 !important;
  text-shadow: 0 0 4px rgba(255,255,255,.75) !important;
  background: rgba(255,255,255,.27) !important;
  background-clip: padding-box !important;
}
#TabsToolbar .tabbrowser-tab:not[pinned] {
  margin: 0 !important;
}
#TabsToolbar .tabbrowser-tab[pinned] {
  height: 26px;
}
#TabsToolbar .tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] {
  padding-top: 4px !important;
}
#TabsToolbar .tabbrowser-tab[selected] {
  background: #EAF2FA !important;
  background-clip: padding-box !important;
  border-color: rgba(0,0,0,.25) !important;
}
#TabsToolbar .tabs-newtab-button {
  -moz-border-top-colors: none !important;
  -moz-border-left-colors: none !important;
  -moz-border-right-colors: none !important;
  -moz-border-bottom-colors: none !important;
  border-style: solid !important;
  border-color: transparent !important;
  border-width: 1px 1px 0 !important;
  border-radius: 2px 2px 0 0 !important;
  text-shadow: 0 0 4px rgba(255,255,255,.75) !important;
  background: transparent !important;
  background-clip: padding-box !important;
  margin: 0 !important;
}
#TabsToolbar .tabs-newtab-button {
  border-width: 1px 1px 0 !important;
  width: auto !important;
  padding: 0 4px !important;
}
#TabsToolbar .tabs-newtab-button:hover,
#TabsToolbar .tabbrowser-tab:hover:not([selected]) {
  border-color: rgba(0,0,0,.2) !important;
  background-color: rgba(255,255,255,.55) !important;
}
#TabsToolbar .tab-background {
  margin: 0 !important;
  background: transparent !important;
}
#TabsToolbar .tab-background-start,
#TabsToolbar .tab-background-end {
  display: none !important;
}
#TabsToolbar .tab-background-middle {
  margin: -4px -2px !important;
  background: transparent !important;
}
#TabsToolbar .tabbrowser-tab:after,
#TabsToolbar .tabbrowser-tab:before {
  display: none !important;
}
#TabsToolbar .tab-throbber:not([pinned]),
#TabsToolbar .tab-icon-image:not([pinned]) {
  -moz-margin-start: -4px !important;
}
.tab-close-button:not([pinned]) {
  -moz-margin-end: -4px !important;
}
#TabsToolbar .toolbarbutton-1 {
  margin-bottom: -1px !important;
}


/* hide scroll buttons on tab bar */
.tabbrowser-arrowscrollbox > .scrollbutton-up,
.tabbrowser-arrowscrollbox > .scrollbutton-down {
  display: none !important;
}
.tabbrowser-arrowscrollbox > .arrowscrollbox-overflow-start-indicator:not([collapsed]),
.tabbrowser-arrowscrollbox > .arrowscrollbox-overflow-end-indicator:not([collapsed]) {
  margin-bottom: 0 !important;
}
/* emulate highlighting of "all tabs" button on new background tab */
.tabbrowser-arrowscrollbox > .scrollbutton-down[notifybgtab] {
  display: block !important;
  list-style-image: url(chrome://browser/skin/toolbarbutton-dropdown-arrow-inverted.png) !important;
  -moz-image-region: auto !important;
  border: none !important;
  padding: 1px 6px 0 !important;
  margin: 0 -46px 0 25px !important;
}


/* Change the big round back button for a smaller rectangular one */
:root #back-button > .toolbarbutton-icon {
  -moz-image-region: rect(0, 54px, 18px, 36px);
  border-radius: 2px 0 0 2px !important;
  padding: 4px !important;
  width: 28px !important;
  height: 28px !important;
}
@media (min-resolution: 1.1dppx) {
  :root #back-button > .toolbarbutton-icon {
    -moz-image-region: rect(0, 108px, 36px, 72px);
  }
}
:root #forward-button > .toolbarbutton-icon {
  padding: 4px !important;
  width: 27px !important;
  height: 28px !important;
}
window:not([chromehidden~="toolbar"]) #urlbar-wrapper {
  clip-path: none !important;
  margin-left: 0px !important;
}
window:not([chromehidden~="toolbar"]) #urlbar-wrapper > #forward-button[disabled] {
  display: none;
}
#notification-popup-box[hidden] + #identity-box {
  padding-inline-start: 5px !important;
}
#notification-popup-box:not([hidden]) + #identity-box {
  padding-inline-start: 10px !important;
}
#urlbar {
  border-left: 1px solid var(--urlbar-border-color) !important;
}
#urlbar[focused] {
  border-color: Highlight !important;
}


/* hide "PanelUI" button */
#PanelUI-button {
  display: none !important;
}


/* hide additional "Bookmarks" button inserted by "Plain Old Favorites" */
#pof-toolbar-menu {
  display:none!important;
}
/* make bookmark toolbar's width tiny */
#personal-bookmarks {
  min-width: 12px !important;
  max-width: 12px !important;
  margin-right: 2px !important;
}


/* don't display click-to-play notification for disabled hidden plugins */
notification[value="plugin-hidden"] {
  display: none !important;
}