The Full Screen mode setting is stored in the xulstore.json file in the Firefox profile folder.

Try to rename/remove the xulstore.json file in the Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thanks for your answer. I tried your suggestion both deleting and renaming the file, putting firefox in full-screen mode, exiting and restarting it. It did not retain the full-screen setting either time and the file xulstore was recreated each time. Any other suggestions?

You can check the xulstore.json file to see if sizemode":"fullscreen" is saved to the file.

"chrome://browser/content/browser.xul":{
"main-window": {"screenX":"0","screenY":"0","height":"991","width":"1162","sizemode":"normal/fullscreen"}}

The text of my xulstore file follows:

{"chrome://browser/content/browser.xul":{"main-window":{"width":"974","height":"706","screenX":"337","screenY":"101","sizemode":"maximized"},"navigator-toolbox":{"iconsize":"small"},"titlebar-placeholder-on-menubar-for-caption-buttons":{"width":"105.39999389648438"},"titlebar-placeholder-on-TabsToolbar-for-captions-buttons":{"width":"105.39999389648438"},"toolbar-menubar":{"autohide":"false"},"sidebar-box":{"src":"chrome://browser/content/history/history-panel.xul","sidebarcommand":""},"sidebar-title":{"value":""}},"chrome://browser/content/preferences/preferences.xul":{"BrowserPreferences":{"lastSelected":"paneMain"}},"chrome://mozapps/content/downloads/unknownContentType.xul":{"unknownContentType":{"screenX":"749","screenY":"349"}},"chrome://global/content/viewSource.xul":{"viewSource":{"sizemode":"normal"}},"about:config":{"prefCol":{"ordinal":"1","sortDirection":"ascending"},"lockCol":{"ordinal":"3"},"typeCol":{"ordinal":"5"},"valueCol":{"ordinal":"7"}},"chrome://browser/content/places/places.xul":{"placesContentTags":{"ordinal":"3"},"placesContentUrl":{"ordinal":"5"},"placesContentDate":{"ordinal":"7","sortDirection":"descending"},"placesContentVisitCount":{"ordinal":"9"},"placesContentKeyword":{"ordinal":"11"},"placesContentDescription":{"ordinal":"13"},"placesContentDateAdded":{"ordinal":"15"},"placesContentLastModified":{"ordinal":"17"},"places":{"screenX":"185","screenY":"60","width":"700","height":"500","sizemode":"normal"}},"chrome://mozapps/content/handling/dialog.xul":{"handling":{"screenX":"605","screenY":"238","width":"326","height":"348"}}}

I attempted to insert "/fullscreen" after normal at both locations and saved the file, but when I reopened it, the inserted text had not been saved. Any other suggestions would be appreciated.

The content shows "sizemode":"maximized" and not "sizemode":"fullscreen", so the Full Screen setting didn't get stored. There shouldn't be a leading backslash as part of the value

I opened the xulstore file in wordpad and replaced maximized with fullscreen. I then saved the file, placed firefox in fullscreen mode and closed the program. When I restarted Firefox it was maximized again, but not fullscreen and the xulstore file had been rewritten replacing fullscreen with maximized. Do I need some special editor to edit xulstore or am I doing something else wrong?

I've checked it and it looks that the full screen setting is no longer stored in xulstore.json, so you can no longer restart Firefox in Full Screen mode. I'm not sure of the reasoning behind this change, but it is likely a (security) issue where windows can he hidden and a full screen window takes over the entire desktop.

Thanks for the info. Is there anything that can be done or is it something I'm just going to have to live with?

I think that you have to live with this. You can check the Add-ons website to see if there is an extension that allows to start Firefox in Full Screen mode.