My Firefox and Tree Style Tab custom css

tree-style-tab.css

:root {
  --tab-highlighter: transparent;
}

#tabbar {
  background: #eee;
  border: none;
}

tab-item-substance {
  padding: 6px 5px 6px 10px;
}

tab-item {
  border: none;
  border-bottom: 1px solid #ddd;
  font-size: 12px;
  background: #eee;
}

tab-item:hover {
  background: #f6f6f6;
}

tab-item.active,
tab-item.active:hover {
  background: #dbdbdb;
}

tab-item tab-favicon {
  margin-right: 5px;
}

tab-item tab-twisty {
  display: none;
}

/* tab-item[data-level][data-level="1"], */
tab-item[data-level][data-level="2"],
tab-item[data-level][data-level="3"],
tab-item[data-level][data-level="4"],
tab-item[data-level][data-level="5"],
tab-item[data-level][data-level="6"] {
  padding-left: 0px;
}

.newtab-button {
  display: none;
}

userChrome.css

#TabsToolbar {

}

.toolbar-items,
.titlebar-spacer {
  display: none !important;
}

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

/* Windows */
:root:not([uidensity="compact"]){
  --uc-toolbar-height: 38px;
}

#TabsToolbar{ 
  visibility: collapse !important; 
}

#toolbar-menubar{
  min-height: unset !important;
  height: 38px !important;
  position: relative;
}

#main-menubar{
  -moz-box-flex: 1;
}

#main-menubar > *{
  padding: 10px 6px;
}

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