A few things to do here.

You can delete a possible user.js file and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. You can get to the profile folder below:

• Help > Troubleshooting Information > Profile Directory > Show Folder

or

• Go to about:support and click Show Folder next to Profile Directory.

Check to see if any suspicious or unwanted software has been installed without you knowing by going to the Control Panel.

• Start > Control Panel > Uninstall a Program

This article also discusses the user.js file: How to fix preferences that won't save.

Thanks for your help on this problem guys. I did try the control panel option but Conduit wasn't listed.

I did however solve the problem by downloading the free Malwarebyte application and scanning the p.c. It found 129 locations that Conduit had 'infected'!

After the scan I quarantined the infected files and everything is now working fine. Thanks again for such a swift response...