Run the Firefox program once as Administrator (right-click: Run as Administrator).
If that doesn't help then do a clean reinstall.

• Download a fresh Firefox copy and save the file to the desktop.
• http://www.mozilla.com/firefox/all.html
• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data if you uninstall the current version.
• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

You do not need to create a new profile, that is not necessary for this issue

Your bookmarks and other profile data are stored elsewhere (not in the Firefox program folder) and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

Many thanks, I got there in the end, installed older version via custom in a different location and that now works. Couldn't remove the 4 RC version as I get the same message to restart as described above.

All working again now V3.??, but assistance much appreciated.