Spurious "Warning: Potential Security Risk Ahead" error page with no way to proceed
I am trying to access my QNAP NAS configuration page within my home LAN from a newly installed Linux PC with Firefox 147.0.1. Firefox won't let me access the page, popping up an error page:
Warning: Potential Security Risk Ahead
Firefox detected a potential security threat and did not continue to nessy.fritz.box. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details.
I have verified that the claimed risk is nonexistant, but Firefox does not allow me to bypass the warning. There is no "Proceed anyway" or "Advanced..." button as claimed in various help articles.
How can I convince Firefox that I know what I am doing?
NB: The same page is accessible, also with Firefox 147.0.1, from a Windows PC.