May I ask where you are getting the error message from? Also a screen shot will help if you can do that.

It is when I go to windows explorer, users, appdata, mozilla, firefox, default..... Both the Prefs and user folder won't open, so I can't see what the settings are and therefore I can't change the preferences because the error message prevents the folder from opening.

I see that you are on a Windows 8 preview, so make sure that you only run programs that are full compatible with Windows 8.

You can do a disk check with the chkdsk.exe program.
If you run the chkdsk.exe program from a cmd.exe Command window then you can read the response from the chkdsk.exe program.

Open a cmd.exe window:
Start > Run: cmd.exe <press Enter>

At the command prompt (>) type or Copy&Paste: chkdsk.exe /f /r <press Enter> (put a space before /f and /r)

If you get something like: Would you like to schedule this volume to be checked the next time the system restarts? y/n then answer the question with "Y" and close all programs and reboot the computer.

You may have to remove the entire Mozilla or Firefox folder in AppData.

thanks cor-el, I 'll give it a try it's not a big problem, just a nuisance that I can't fix something and change the preferences. It didn't correct the problem. Thanks anyway.