Thank you all for trying to help!
This is v. odd. When I woke up this morning I had an idea about this, but before I tried it I went to that pesky page anyway. To my amaze it was perfectly ok and as it should be. All that had happened on that Mac was an overnight reboot.
Turning to this Mac (there are four), I rebooted, then went to the pesky page. It was all over the place, the same as yesterday.
That makes no sense whatsoever!
Then I had a look at Firefox preferences, and clicked on the Advanced tab by JavaScript. I clicked on the only thing which was unclicked: Allow scripts to raise or lower windows, clicked ok. Back to pesky page, refreshed.
Ta Daaaa!! Sorted.
There's a lesson in this somewhere. All I can think of, is Tesco is the only website I've ever used which demands that JavaScript be enabled in that particular way.
