Thank you, SD. I do have Adblock Plus running. About a year ago, on my now extinct W98 machine, I tried NoScript and it appeared to cause some disaster the details of which escape me now. Was that unusual? Perhaps it was the W98's limitations.
A bit of research in the meantime suggests that NoScript retains problematic aspects, especially in the context of its relationship with other useful software - favouring some and prejudicing others - which brings me back to my unhappiness about things happening behind my back..... Is there no other way to stop my use of Firefox being hijacked?