Perhaps fullscreen is not set to hide your toolbars and stuff while in fullscreen? Please try the following to enable this feature:

1. Go to about:config in the Firefox address bar
2. Click the I'll be careful button
3. Locate the browser.fullscreen.autohide boolean
4. Change the value to true

Let us know if this fixes the issue.

Note: You may need to restart Firefox for the settings to take effect.

You can right-click empty toolbar area to select whether to show or hide the Navigation Toolbar and the Tab bar in Full Screen mode.

• https://support.mozilla.org/kb/how-to-use-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" menu button drop-down list
• right-click empty space on a toolbar and click "Exit Full Screen Mode"