Hello,

This setting is stored in the localstore.rdf file in the Firefox profile folder.

• http://kb.mozillazine.org/localstore.rdf

You you can check this file to see if sizemode="fullscreen" appears under the browser.xul#main-window heading.

If have this section when the normal dimensions are 1024x720.

<RDF:Description RDF:about="chrome://browser/content/browser.xul#main-window"
     height="720"
     width="1024"
     screenX="323"
     screenY="90"
     sizemode="fullscreen" />

From: questions/966532

Firefox will open in the Full Screen mode if you close it while in Full Screen mode. At least on Windows OS's; you'll need if it;s the same to see on Mac.

That feature was added back around Firefox 4 or 5.

You should make sure to use "Firefox > Quit" to close Firefox while still in full screen mode and not close the current window and still have the Firefox menu bar open.