Could you visit my test page, which I think will help yield more diagnostic information:

https://jeffersonscher.com/res/jstest.php

Expand the "Technical Details" section of the error page. What Error code do you find (in parentheses)?

If it's an unknown issuer error:

Expand the "I understand the risks" section and look for an Add Exception button. You don't need to complete the process of adding an exception (I suggest not adding one until we know this isn't a malware issue) but you can use the dialog to view the information that makes Firefox suspicious.

Click Add Exception, then View. If View is not enabled, try the Get Certificate button first. Then in the Certificate Viewer, look at the "Issued by" section. What do you find there, and/or under Certificate Hierarchy? I have attached a screen shot for comparison.

Microsoft Family Safety

Many users upgrading old systems to Windows 10 have encountered issues with Microsoft Family Safety. They accidentally apply parental controls to themselves. You can try disabling it (or not being part of a family): http://windows.microsoft.com/en-us/windows-10/turn-off-microsoft-family-settings

Alternately, if you like that filter, you can import its certificate. See: https://support.mozilla.org/questions/1083592#answer-781104

Thanks, this is the error code I received when opening your link in firefox:

jeffersonscher.com uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. (Error code: sec_error_unknown_issuer)

oops, forgot the screenshot of the cert detail

Yes, that's not the real site certificate because instead of being signed by Comodo it is signed by "Content Watch Authority". Judging from the URL listed on the dialog, it sounds like this product: https://www.contentwatch.com/solutions/needs/internet-filtering

Can you access my test page without problems in either Edge, IE, or Chrome? If so, you have the Content Watch Authority certificate saved in the Windows certificate store. You can export it from there and import it into Firefox. I haven't tried it on Windows 10, so apologies if this doesn't match what you see there. Hopefully it's still in the ballpark.

Export

• Open a secure site in IE or Chrome and click the padlock icon on the address bar. On Chrome, click Connection to find the link to View Certificate. (On IE, I forget the exact steps.)

• In the IE or Chrome certificate viewer, switch to the the Certification Path tab. Click the cert you want to export and use the View Certificate button to open it directly. Note: Try exporting the top level Content Watch certificate at the top of the path.

• Then click the Details tab and click the Copy to file button. This starts the Export Wizard. Use the DER format and save to a convenient location.

Import

• In Firefox, open the Certificate Manager using: "3-bar" menu button (or Tools menu) > Options > Advanced > Certificates mini-tab > "View Certificates" button

• Click the Authorities mini-tab and then the "Import" button, and find the DER file. Note: I suggest allowing the certificate for websites only unless your IT suggests otherwise.

I am attaching some screen shots for reference.

Does it work?