Hi Craig,

This is to give you an update of the situation: I am pleased to say that the issue is now resolved and was due to corruption within Firefox possibly caused by an update not functioning completely correctly.

To solve the issue, I first uninstalled the Avast Browser Protection, and then did a complete uninstallation of Firefox including all personal settings (I used Revo Uninstaller for this to get rid of related registry remnants that were left behind by the Firefox uninstaller). Fortunately I had a backup of my Firefox profile created some time before the problem first occurred which meant I was able to reinstate most of my required bookmarks at once with only a minimum of fiddling to get back to my position immediately before the Firefox problem. I then reinstalled the Avast Browser Protection extension successfully.

Thanks for your help. Although it did not provide an immediate solution, it did enable me to confirm Firefox itself was the problem and after sorting Firefox out, your advice was very useful. Happily we can now say that the issue is resolved.

Best regards, Kes.