This feature in Firefox allows users to securely store credentials – the following Mozilla Support articled describes how to implement and resolve issues with this feature:
If you choose to reset your master password, this procedure deletes any existing security credentials and passwords as a security measure. If you have existing security certificates, you will haveto re-import them after you reset the master password.
If you delete the master password, you can then create a new Software Security Device as part of thecertificate download process. If you already have a personal certificate, you should Import Certificates to the Firefox Certificate Manager.