I used to have my tabs hidden (I used Tree-Style too) but now they won't hide. Could you share your CSS you are using?

Mine looks like:

@-moz-document url("chrome://browser/content/browser.xul") { #TabsToolbar { visibility: collapse !important; } }

hey, sorry for late reply, i know some ways have stopped working over time, so my userChrome has accumulated a bunch of them:

#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items { opacity: 0; pointer-events: none; }

#main-window:not([tabsintitlebar="true"]) #TabsToolbar { visibility: collapse !important; }

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

#titlebar { visibility: collapse; }