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

Thanks. That seems to have done the trick. Let me try it for a day or two and then if all's OK i'll click the "Solved the problem" button on this page.

OK - deleting/renaming the xulstore.json file solves the problem until the next day when the full screen stops working again. So not a solution.

The xulstore.json file shouldn't get corrupted in a way that you can't exit full screen mode.

What security software do you have?

There is software like Advanced SystemCare with its Surfing Protection feature that can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Windows Defender. No, i don't have Advanced System Care.

Another thing: the bookmarks bar is just blank whenever I open Firefox and i have to go into View | Toolbars, de-select it and then re-select it again to make it visible.

I would suspect a slightly incompatible add-on as causing that issue.

Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.