TDN uses certificates from "Let's Encrypt!" JellyNeo may, too, but I'm not positive. Yesterday, the "DST Root CA X3" certificate expired (30 September 2021). That root certificate was used by "Let's Encrypt!" when they first started, and root certificates are used in/by web browsers to trust other certificates issued by them. For example, TDN gets a certificate from "Let's Encrypt!". Your web browser has the "DST Root CA X3" certificate installed. Therefore, your web browser will "trust" TDN's certificate, because of the "DST Root CA X3" certificate. Here is information from "Let's Encrypt!" about their certificate expiring, which was intended:
https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
The Brave browser is derived from Chrome, and, now, so is Microsoft Edge. Your Windows PC has a centralized certificate store. Chrome uses that certificate store, instead of having its own. The "DST Root CA X3" is most likely in your Windows centralized certificate store. (I just checked, and it is in mine). Because the certificate is expired, is why Brave and Chrome give those error messages.
On the other hand, Firefox doesn't use Windows centralized certificate store. It uses its own certificate store, built into Firefox. Because Pale Moon is a Firefox derived browser, it does the same. If you look in Firefox, you will most likely find the "DST Root CA X3" certificate in there. However, Firefox, also, has the new "ISRG Root X1" certificate installed, where my Windows PC certificate store doesn't. Because it has the new certificate, is why the sites work in Firefox and Pale Moon.
The issue was supposed to be transparent, meaning sites were not supposed to "break" (except for older devices) after 30 September 2021, but that doesn't appear to be the case. Well, they didn't for Firefox. Your Windows PC should receive an update, with the updated certificate. Or, at least, that's what I believe will happen. Until then, this site has an explanation and fix:
https://www.stephenwagner.com/2021/09/30/sophos-dst-root-ca-x3-expiration-problems-fix/
The "fix" may not be clear. Right now, it is late, for me, but I will post instructions tomorrow on how to install certificates in Windows. Hope this helps.
If you are on Windows 10, check to see if there are any Windows Updates available. Mine just found one. I'm currently downloading and installing it. I'll see if it fixes the issue after a reboot. If it does, I'll post to let everyone know.
By the way, TDN's "Let's Encrypt!" certificate is still valid/not expired. If you are receiving the "not secure" message, it is due to the "DST Root CA X3" certificate being expired.