It means (most likely) that your antivirus has changed some of Firefox's preferences. You can enter about:policies in the URL bar to see, what's going on.

Thanks. I have now looked at that page, and it does not contain anything that makes sense to me. You are probably right that it is something to do with AntiVirus. Why do they use such confusing terminology? It is nothing to be with an organisation.

Having found one or two other posts about the problem it looks as though one might be able to sort it out by fiddling around with the registry. I hate tinkering with the registry, and I wonder if some clever person could provide a tool for sorting this out automatically.