You have asked Firefox to connect securely to cacert.org, 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.
I'm sure e12e knows this. If you don't want to see that message, you need to install the CAcert root certificate in your browser or OS. While you're doing that, take a look at all the root certificates that are already in there: you might decide CAcert is more trustworthy than most of those organizations!
https://bugzilla.mozilla.org/show_bug.cgi?id=215243
For some more background, I suggest reading:
http://www.thoughtcrime.org/blog/ssl-and-the-future-of-authenticity/
http://wiki.cacert.org/InclusionStatus