This error message came up when you attempt to open an https website using Mozilla Firefox browser. The Mozilla Firefox browser will verify the security certificate of the site before making any connection between it and the server, and then display the website.
And due to any reason, the certificate is rejected, then the Mozilla Firefox browser will show an untrusted error as you have right now, warn you about a security risk. In most cases, you can ignore this warning and still access the site.
The error comes due to many technical details, such as an incorrect computer clock, a corrupted SSL certificate file or even the certificate is expired. So, in order to fix this problem on your Mozilla Firefox browser, you can follow the instruction video on here or follow the tutorial in the link to fix or ignore the This Connection is Untrusted error.