SEC_ERROR_UNKNOWN_ISSUER when https in Firefox

When you connect to https website in Firrefox, you may get an error: you connection is not secure and SEC_ERROR_UNKNOWN_ISSUER error.

This is due to that Firefox background ssl certificate process is not updated. We can remove temporary Firefox storage to fix the problem. The process is as following:
 
1) Click Any folder in Windows
2) Click Tool->View at top menu of the folder.
 3)  Check "Show hidden files, folder and drive" checkbox
4) Go to C:\Users, click your user name
 5) Backup Firefox bookmark
6) Go to AppData\Local\Mozilla, remove Firefox folder
7) Go to AppData\Roaming\Mozilla, remove Firefox folder
 8) If  folder AppData\LocalLow\Mozilla exsits, go to AppData\LocalLow\Mozilla and remove Firefox folder.