I got the following error message on Firefox.
xxxxxx uses an invalid security certificate.
The certificate expired on 21/12/2010 11:59 AM.
(Error code: sec_error_expired_certificate)
I found from online sources that a very quick fix to this was to temporarily change the date on the computer. And I did just that. I changed the system date on my PC from 21 December 2010 to 21 November 2010.
Having done that I refreshed my Firefox and to my delight the error message was gone. I then switched back to the current date.
So that's a really quick and very effective way to fix the invalid security certificate error.