If it only happens on one site then that might rule out the adware possibility. It could be a deliberate change by whoever operates the site, to try monetise it?

Thanks for the reply Darren but it does not occur if I use Microsoft Edge. It started after the last Firefox update I think.

I know Edge has a pop-up blocker by default but not an ad blocker unless you install one. Maybe post the link here and others might have input on what that site might be doing? I still doubt it is adware on your computer if it only happens on one site, but it might be worth doing a full scan with up-to-date antivirus/antimalware software anyway, just in case.

OK solved. I started getting other sites redirecting me to advertising sites. Following your advice Darren I checked my Ad blocker and found it was turned off. How I do not know. Possibly after an update it loaded in a default state. Have turned it on and AdBlocker Ultimate has stopped the redirects. Thanks Darren for you help