Why do I keep being prompted for my "Master Password"?
If Firefox is repeatedly asking for your master password for the Software Security Device, even after you have entered it, it means that the password you entered is incorrect. Make sure that you are typing the password correctly and that the Caps Lock key is not pressed.
To change or remove your master password for Firefox, see this KB article on support.mozilla.org.
If you still encounter problems, you will need to reset your certificate profile.