I read through all the archived comments on previous posts and NONE of them worked for me. So if that describes your experience then here is a simple solution:

"C:\User\YOURUSERNAME\AppData\Local\Mozilla\updates\EFCF176E110C211B\0" Change the very last character from "0" to 1. That's it! Your problem is gone.

This problem is now solved

I tried this and had to change EFC...211B to the string in my case so it read: C:\Users\Owner\AppData\Local\Mozilla\updates\308046B0AF4A39CB

This was an empty file so using \0 (to start from the same pace as the advice) or \1, all I got was an error :" Windows can't find 'the string above' Check the spelling and try again".

How do I insert \0 or \1 on the end of a file name? Answer: I can't; \ / are forbidden characters. Back to square 1.

Check that Firefox isn't set to run as Administrator.

You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab.

Make sure all items are deselected in the "Compatibility" tab of the Properties window.

• Privilege Level: "Run this program as Administrator" should not be selected
• "Run this program in compatibility mode for:" should not be selected

Also check the Properties of the firefox.exe program in the Firefox program folder.