If you see SEC_ERROR_REUSED_ISSUER_AND_SERIAL warning in your Firefox browser, then you need to take a few steps to resolve the issue.

Your Connection is not secure is one of the common problems on Firefox & in order to troubleshoot, we need to understand the type of Firefox connection errors.

Firefox: 'Secure Connection Failed' to Microsoft.com When using Firefox, accessing microsoft.com is not working quite as expected for many around the world.

After following the solutions mentioned, you will successfully fix PR_CONNECT_RESET_ERROR On Mozilla Firefox in Windows 10.

After having tweaked Firefox to only use TLS version 1.2, one website can verify that the tweak worked and another can test it.