My current Firefox UI modifications via userChrome.css

userChrome.css

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

/* Hide horizontal tabs at the top of the window */
#tabbrowser-tabs {
  visibility: collapse !important;
}

/*Collapse in default state and add transition*/
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] {
  overflow: hidden;
  min-width: 1px;
  max-width: 1px;
  transition: all 0.2s ease;
  border-right: 1px solid #345f0c !important;
  z-index: 2;
}

/*Expand to 200px on hover*/
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]:hover,
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar {
  min-width: 200px !important;
  max-width: 200px !important;
  z-index: 1;
}

#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
  display: none;
}

/* Hide splitter, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] + #sidebar-splitter {
  display: none !important;
}