Many thanks for the replies, since my first post I updated my Macbook Air & Pro to Big Sur. Version 11.2.1, and both now have the same problem with Barclays Online Banking. I tried them both on Chrome and firefox but still get the same problem. However I have since tried Safari and it works fine.
My conclusion is the Barclays Online Bank software is probably the problem, but I cannot rule out that Chrome & Firefox current versions are not totally compatible with Big Sur, as they both worked fine using Catalina.
To confirm after login in fine, if I selected any page from the menu I got a blank page with the following error message:
Error 500: javax.servlet.ServletException: Filter [securityFilter]: filter is unavailable.
I sent a support ticket to Google, but as expected they never replied. Barclays were also not very helpful suggesting I use the Mobile app to access the online banking.
As safari works OK, that is what I will use until this is sorted.
Thanks again for your suggestions.