“this connection is untrusted”

"(Error code: sec_error_unknown_issuer)"

The fix for this is to save and then import the Microsoft Family Safety Certificate into the browser.

Log in to the computer using the affected account.

Open the run command (Windows key + R, it may also appear as an option on your start menu, or you can open the charms menu and search for “run”)
In the Run dialogue box, type certmgr.msc to open the Certificate Manager window.

Alternatively, you can access the Certificate Manager through the search charm, search for “certificates” then select “Manage user certificates”.

Once the Certificate Manager window is open, expand Trusted Root Certification Authorities on the left, and click on the Certificates folder.
On the right pane, scroll down to find the Microsoft Family Safety Certificate, right-click it, then All Tasks > Export.

The Certificate Export Wizard will open.
Click “No” to export the Private Key
Select DER encoded binary X.509 (.CER).
Save the file to your preferred location, such as Desktop. The wizard automatically adds a “.cer” extension to the file name.
Click Next, and then click Finish to complete the export.

In Firefox, select Options from the menu.
Select the Advanced icon, and then select the Certificates tab.
Click View Certificates to open the Certificate Manager, and then click Import.
Browse to the location of the certificate file, and then click Open.
In the Downloading Certificate dialog box, select “Trust this CA to identify web sites”, and then OK.
Click OK again to exit the Certificate Manager.