The download site for Tux of Mathe and Tux typing is flagged (by Firefox) as untrusted.

Full warning message:
This Connection is Untrusted
You have asked Firefox to connect
securely to, but we can't confirm that your connection is secure.

Normally, when you try to connect securely,
sites will present trusted identification to prove that you are
going to the right place. However, this site's identity can't be verified.
What Should I Do?

If you usually connect to
this site without problems, this error could mean that someone is
trying to impersonate the site, and you shouldn't continue.
Technical Details uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is not trusted.

(Error code: sec_error_untrusted_issuer)
I Understand the Risks