Firefox opens with the same screen dimensions when you last closed Firefox.

If that isn't the case then there might be a problem with the xulstore.json file.

Try to rename (or delete) 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: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

I tried renaming the xulstore.json file and firefox just created a new one. I tried deleting it and firefox just created a new one. Firefox still will not open in full-screen mode. Do you have any other suggestions?

Are you saying that when you close Firefox while in Full Screen mode it doesn't open the next time in Full Screen mode?

Yes, That is what I am saying.