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After updating my Firefox to version 58, I found that I could not open several pages such as Google.com, Twitter.com, Facebook.com and so on. I received the error – Your connection is not secure, SEC_ERROR_UNKNOWN_ISSUER_ISSUER. If you get this error, there are two ways to handle it.
SEC_ERROR_UNKNOWN_ISSUER Firefox Error
1] Mozilla recommends that you solve this problem by disabling SSL scanning in your security software such as Avast, Bitdefender, ESET and Kaspersky. You must therefore check this parameter in your security software. Let me give Kaspersky as an example.
Open your Kaspersky software and click Settings. You will see the symbol at the bottom left.
Select Additional > Network.
On the Network Settings page, under Scan Encrypted Connections, select Do not scan encrypted connections .
If you do this, it leaves your system a little dangerous because it reduces your computer’s protection, because modules like Parental Control, URL Advisor, Private Browsing, Web Anti-Virus, etc. are not able to control encrypted connections.
In Avast, open the dashboard, go to Settings > Active Protection and click Customize next to Web Shield. Disable the Enable HTTPS Scan setting.
Bitdefender users must click Modules > Web Protection > Disable SSL scanning.
ESET users should disable and re-enable SSL/TLS filtering or generally disable secure connection interception.
This list was only indicative and you must do it for your security software.
2] The other option is to add an exception if the website allows it.
On the Error or Warning page, click Advanced.Then click Add Exception. The Add Security Exception dialog box opens.You can click Get Certificate then click View to view the certificate.
Once you are ready, you must click Confirm the security exception if you are sure you want to trust the site.
Restart Firefox and see. The warning no longer appears.
Believe it works for you.
Read more read : How to fix time-related bugs on secure websites for Firefox.