visibility: collapse;

Hello, I am looking for a way to hide my tab bar when there is only one tab open. I almost found the solution there: https://support.mozilla.org/en-US/questions/1297983 But it is not exactly what i am looking for. This solution hides the tab but not the tab bar it self. I mean it leaves some black empty space because i use dark theme. I kept looking for a solution to hide the container. I found this one: https://support.mozilla.org/en-US/questions/1271391 The problem is: this solution hides all the time the tab bar. No maters how many tabs opened. I experimented mixing these two solution. I have try: #tabbrowser-tabs tab[first-visible-tab="true"][last-visible-tab="true"] + #mainwindow:not([tabsintitlebar="true"]) #TabsToolbar { visibility: collapse; } I tried the other way around, with "#mainwindow" first. I tried without the "+" sign. But none worked out.