It sounds like you may have a feature enabled in Firefox called "Autohide" or "Autohide Toolbar" that causes the top section (such as the address bar or tabs) to hide when not in use. To disable this feature, you can follow these steps:

1)Click the three horizontal lines in the top right corner of the Firefox window to open the Firefox menu. 2)Click "Customize Toolbar" near the bottom of the menu. 3)Look for a checkbox labeled "Autohide Toolbar" or something similar, and make sure it is unchecked. 4)Click "Done" to save your changes and exit the customization mode.

This happens when Firefox is set to run in Full Screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

• https://support.mozilla.org/en-US/kb/how-make-firefox-and-websites-go-full-screen

When you are in Full Screen mode, hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear. You can use one of these to leave Full Screen mode:

• press the F11 key
• click the Maximize button at the right end of the Tab bar
• click the Full Screen button in the "3-bar" Firefox menu button drop-down list
• right-click empty space on a toolbar and click "Exit Full Screen mode"