Fix Firefox connection error for missing certificate

Hi, I am using Waterfox (a Firefox clone) on Windows Server 2012 with rotating residential proxy servers. Depending on the quality of the proxy server (around 40% of proxies), I am unable to connect to any website. Waterfox shows me "Your connection is not secure" Error code: "SEC_ERROR_UNKNOWN_ISSUER" - this happens to every website, and disappears using another proxy, so I am sure the problem is the residental proxy. Maybe an AV or firewall on the computer I connect through, which blocks the Internet connection. If I add an exception, the website works well again. But I need to add an exception for *every* website that I want to visit and this is very time consuming. I would need a global fix, so that Waterfox can connect again. Maybe a certificate database? Issue is not urgent, it's ok if the solution takes a few days. I would prefer to pay a fix budget, not by hour. Thank you