Hi Rui!
Thank you so much for getting back to me with another quick reply and further information. With your help I managed to track down the source of the problem. The one common plugin on both browsers is Ghostery, which had automatically updated to the latest version and broke the website.
For your information, and in case anyone else has the issue. The culprit is the ‘Google AJAX Search API’ – once that is unblocked or if the website is whitelisted, it all works perfectly once again.
Thanks again for all of your help, I hope the information helps you and others.
John