How to turn off Enhanced Tracking

While there are help articles for both disabling ETP and downgrading Firefox, none of these would be really relevant for you. What you're describing is a HTTPS connection error, and to help with that the actual address and full error or a screenshot incl. any "advanced" sections expanded would have to be attached here. The context is in [[Troubleshoot security error codes on secure websites]] esp. the sections about security software installed. Few types of SSL errors are listed in [[Secure connection and security warning error pages in Firefox]] … some are recoverable, some not.