Run the Firefox program once as Administrator (right-click: Run as Administrator).

If that doesn't solve the problem then do a clean reinstall and remove the Firefox program folder.

• http://kb.mozillazine.org/Software_Update (Software Update not working properly)
• How to fix the Update Failed error message when updating Firefox

Clean (re)install:

• 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.

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.

Thanks for your help. I have now made a clean reinstallation and will wait for the next software upgrade and see what happened. Finn

Unfortunate this new upgrade gave the same problem after the clean installation.