If you click the close 'X' in the top right corner then you only close this specific window, other windows will still be open. Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows open to prevent losing tabs in unnoticed windows. *"3-bar" menu button -> Exit (Power button) *Windows: File -> Exit *Mac: Firefox -> Quit Firefox *Linux: File -> Quit