“This Connection is Untrusted”

When I try to log in to my blog at wordpress.com, Firefox tells me:

You have asked Firefox to connect
securely to en.wordpress.com, 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.

en.wordpress.com uses an invalid security certificate.
The certificate is only valid for wordpress.com
(Error code: ssl_error_bad_cert_domain)

What does the above mean? What should I do to access my blog safely?

–Georgs, on MacBook Pro, OS X 10.5.8