Thank you for your time on this. I went to Mac Preferences, then Security & Privacy but got a message as follows:
Preferences Error
Could not load Security & Privacy preferences pane
When I got into Firefox Preferences/Privacy I couldn't work out what needed doing - I looked at all the Permissions but that didn't seem to be what you described.
I'd be very grateful if you could spell it out again, for one who rarely delves beneath the surface of such things.