Hello,
with the newest firefox, access to that web page is possible for us. For us to start diagnosing the issue, we would need a screenshot of the error firefox displays, and if it is an issue with certificates, on the error screen click Advanced, and if there is something like: Error code: SSL_ERROR_BAD_CERT_DOMAIN, click on the SSL_ERROR_LINK and send us the displayed information.
As a note, it is indeed possible to create exceptions for specific pages: Preferences -> Shields -> click on Settings in Web Shield section, in section Excluded Servers click on +, select Service https, set *.nexusmods.com into Server, click Add.
Kind regards,
Ondrej Kolacek