I think I have solved my problem.
I got an exception from
Safari which suggested "External Modification Warnings: Thread creation by external task."
I found many references on the web (most for Windows) and some suggesting going to
Safari\Preferences\Extensions and turning off. I wasn't even going to try those which suggested I install yet another app from the web.
Seeing as I could not even start
Safari the above was not useful. I could start
Safari in Safe mode, but there was nothing there - possibly because when it originally wanted access to all my private data I refused.
Activity Monitor showed an
Installer and this had
~/Library/Application Support/com.genieoinnovation.Installer/Completer.app/Contents/MacOS/Installer open.
I deleted this directory. Unfortunately this did not fix the problem.
I tried @Buscar웃 suggestion, but this only found references to old preferences in Firefox and Chrome, which I had already fixed.
I then tried
sudo mdfind com.genieoinnovation and found the following.
Deleting all these seems to have resolved the issue.