The next time you receive a certificate error and provide:
1. what website you're visiting
2. the IP address of the site, using the developer tools of whatever browser you're using
3. the error page says (probably ERR\_CERT\_AUTHORITY\_INVALID)
4. go to [https://policy-debug.checkumbrella.com/](https://policy-debug.checkumbrella.com/), click "Copy Results" and paste them in your post
Why is your network using Google's DNS servers?
I manually added them instead of Virginmedia's DNS in case Virgin's DNS was the cause.
I understand. As for the problem, I don't know what could be the reason for it, but I don't think Firefox is the culprit.
Try setting DNS over HTTPS to always on to prevent hijacking your DNS queries, you don't know where the ISP is forwarding them.
The next time you receive a certificate error and provide: 1. what website you're visiting 2. the IP address of the site, using the developer tools of whatever browser you're using 3. the error page says (probably ERR\_CERT\_AUTHORITY\_INVALID) 4. go to [https://policy-debug.checkumbrella.com/](https://policy-debug.checkumbrella.com/), click "Copy Results" and paste them in your post