I have tried the various suggestions concerning Firefox saving windows settings including deleting xulstore.json, doing a full restore and total fresh install. After doing any of those, Firefox will open in a full (normal) window. After exiting and reopening Firefox, it goes back to opening in a half screen window. I am running Windows 7 Pro. No other browsers that I use have this issue. Firefox was okay until I upgraded to version 66 and beyond. I haven't gone back to version 65 to see if the issue exist there as well. I do run a modified pref.js that blocks and stops items based on my preferences. The window settings problem exists even if I would do no modifications to the pref.js file.