Hello and thank you, Mat, Iggy and Greybeard. Excellent response.
Mat, I have learnt more since yesterday's post when Ryan, a very helpful guy, from the Disqus help centre advised me that problems tended to arise with the amount of obtrusive code in certain advertisements, which is what you, Iggy, picked up on. Ad block is loaded so I'll look at excluding the Telegraph as you suggest, but whether Firefox has a similar exclusion system I don't know.
Clearly, the permissions repair was unnecessary. For future incidents I'm advised, (a) clear the safari cache, (b) refresh the page, and advise Disqus if particular adverts are present when the problem next occurs.