Here is the procedure to install root certificates on Windows.
Firefox does not trust the same certificates as Windows and a setting must be applied to enable this.
Please consult Mozilla's knowledge base article.
Open the Microsoft Management Console by searching for MMC in the Start menu. Another way is to press the Windows+R keys on your keyboard and, in the Run window, search for MMC.
Select File – Add/Remove Snap-in.
In the Available snap-ins box, select Certificates.
Click the Add button.
In the wizard, select Computer Account, then click Next.
Select Local computer, then click Finish to end the wizard.
Click OK to save your changes. This action will also close the Add or Remove Snap-ins dialog.
In the console, select Certificates (Local Computer).
In the Logical Store Name box, right-click Trusted Root Certification Authorities and select All Tasks – Import.
Follow the instructions in the Certificate Import Wizard and provide the certificate file you have.
Here is the procedure to install root certificates on Linux (Ubuntu and Debian).
Firefox does not trust the same certificates as Windows and a setting must be applied to enable this.
Please consult Mozilla's knowledge base article.
sudo mkdir /usr/local/share/ca-certificates/extra
sudo cp ca.crt /usr/local/share/ca-certificates/extra/ca.crt
sudo update-ca-certificates
Watch for a "1" added in the output messages.
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