It took me a while to figure it out from googling, but the solution is simply to delete the cert8.db from your FireFox profile, usually found in /home/hafiz/.mozilla/firefox/t5q6cggf(some-name).default and Easier still just search for cert8.db in your computer and delete that file. Close and Start again your firefox
Windows:
It took me a while to figure it out from googling, but the solution is simply to delete the cert8.db from your FireFox profile, usually found in C:\Documents and Settings\<yourname>\Application Data\Mozilla\Firefox\Profiles\<yourprofile>. Easier still just search for cert8.db in your computer and delete that file.
Oh….. you need close FireFox first
Got this information somewhere in net and worked fine for me.