The following error occured several times with my Firefox:
Website Uses an invalid security certificate
The certificate is not trusted because the issuer
certificate has expired.
I searched over the internet for a while with no result.
But at last i found out that the error was with my system time.
As because my Mother Board Battery was not functioning well, it was setting the time always to the dafault i.e. 1-Jan-2000.
I simply changed it to the current time, and everything worked fine.
Again i replaced the Motherboard Battery.