Firefox userstylesheet, tabs under nav-bar.

Firefox Tabs Like Safari.css

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

@-moz-document url("chrome://browser/content/browser.xul"){

#titlebar{
  margin-bottom:0 !important;
}

#appmenu-button-container{
  min-width:98px !important;
}

#TabsToolbar{
  margin:0 -4px 0 -4px !important;
  -moz-box-ordinal-group:101 !important;
}

tab,
.tabs-newtab-button,
.tab-content{
  -moz-transform:scaleY(-1) !important;
}

#main-window[sizemode="maximized"] #nav-bar{
  border-top:1px solid rgba(26,26,26,0.5) !important;
}

toolbar:not(#TabsToolbar):not(#toolbar-menubar):not(#addon-bar){
  border-bottom:1px solid  rgba(26, 26, 26, 0.4) !important;
}

toolbar:not(#toolbar-menubar){
  margin-bottom:-1px !important;
}


#main-window[sizemode="maximized"][tabsontop="true"] #TabsToolbar{
  margin:0 -126px 0 -103px !important;
}

#main-window[sizemode="maximized"][tabsontop="true"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar,
#main-window[sizemode="maximized"][tabsontop="true"] #titlebar[hidden="true"] ~ #tab-view-deck #TabsToolbar{
  margin:0 !important;
}

#nav-bar{
  -moz-box-ordinal-group:100 !important;
  background-image: linear-gradient(rgba(255, 255, 255, 0), rgba(255, 255, 255, 0.5))!important;/*firefox 16++*/
  visibility:visible !important;
}

/*forward button*/
#nav-bar[currentset*="unified-back-forward-button,urlbar-container"] #forward-button[disabled="true"]{
  visibility:hidden !important;
}

toolbar:not(#toolbar-menubar):not(#TabsToolbar):not(#addon-bar):not(#nav-bar){
  -moz-box-ordinal-group:100 !important;
  background-image: linear-gradient(rgba(255, 255, 255, 0.5), rgba(255, 255, 255, 0.5))!important;/*firefox 16++*/
}

#main-window[sizemode="maximized"] #nav-bar{
    border-top:1px solid rgba(26,26,26,0.5) !important;
}

toolbar:not(#TabsToolbar):not(#toolbar-menubar):not(#addon-bar){
  border-bottom:1px solid  rgba(26, 26, 26, 0.4) !important;
}

.tabs-newtab-button image[class="toolbarbutton-icon"]{
  margin-top:-2px !important;
}

}