Question Author
Thank you for your response and apologies that I haven't acknowledged sooner. AnswerBank took ages and ages to download onto my screen and I gave up waiting. I seem to have had a few problems. When I clicked on Tools > Add-ons > Extensions > Options, this opened a dialogue box listing My Exception Rules and My Adblocking Rules. There were boxes with ticks and I could delete those items, but the main accumulation of several pages did not respond in that way - no boxes or ticks. One thing led to another and in the end I uninstalled and reinstalled Firefox. Everything's OK now, with a fresh start, but I would like to know where I went wrong!!