You can modify your userChrome.css file to hide the tabs and close window button.

Firstly, create a userChrome.css file. Refer to the link below for step-by-step instructions. How to Create a userChrome.css File

Secondly, for the CSS code, refer to jscher2000's solution in a previous thread below. Remove Tabs on Top

Hi strafy, Thank you for your help. I create userChrome.css and enable toolkit.legacyUserProfileCustomizations.stylesheets. Then, I use the code from the link "Remove Tabs on Top". But it still display as normal firefox browser.

Hi strafy, My firefox version is 78.9. After I remove the line "@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");", firefox will not show tab bar and close button!! Thank you!! But the GNOME top-bar is still there. And, there is a "quit" in the menu. How to remove menu button?

Hi strafy, My setting in userChrome.css is as following.

/* Hide all tabs and tab bar buttons */ #main-window:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {

 opacity: 0;
 pointer-events: none;

}

/* This makes the bar shorter vertically but removes the 3 window buttons */ #main-window #TabsToolbar {

 visibility: collapse !important;

}

/* For full screen mode */ #TabsToolbar[inFullscreen]{

 visibility: collapse !important; 

}

/* Hide SearchBar */ #urlbar {visibility: collapse;}

/* Hide Hambergur Menu */ #PanelUI-menu-button {display: none;}

Right now, it still has 3 problem. 1. still can't open firefox in fullscreen mode 2. In this way, it is not like Kiosk and there is the right-click menu. How to disable right-click menu?

Thank you.