The TLS certificate is self-signed, so the warning is "correct". The encryption is just as strong as any other HTTPS connection, but without a CA's signature the site's identity can't be guaranteed -- a man-in-the-middle attack could be used. Unfortunately, getting a signature costs money.

Verifying the certificate's fingerprint here before adding an exception will prevent any MITM attack. This is only necessary when you add an exception; subsequent MITM attempts will trigger a warning by your browser.

The CA's root certificate needs to be included in the browser to make the warning go away. CACert isn't included in any popular browser, and Startcom was only recently added to Windows. With Startcom, anyone who doesn't install the optional root certificates update in Windows update will still get an error in Chrome, Safari, and Internet Explorer. Firefox has had it built-in for a while.

HTTPS isn't the default (or really necessary) for bitcoin.org, so it doesn't much matter. If you want to manually switch to HTTPS, then you can deal with the self-signed certificate.

No need to worry for this kind of protection from firefox. Since BTC is now being adapted globally, not all would be affected of this kind of feedback.