This could be due to the AVG WebShield feature. I am not familiar with where that is in AVG's control panel(s) but it might be similar to Avast.

How to troubleshoot security error codes on secure websites

Thanks for the response jscher2000. Sorry for my late reply.

I uninstalled WebShield, but continued to receive the error. I then uninstalled the email component and then completely uninstalled AVG. Once I did that, I restarted my laptop and launched Firefox (with no anti-virus on at all). I then tried to load Google and Gmail. Same error on each site.

Did anything else in the article seem relevant to your system? If not, let's take a look at the certificate that Firefox objects to.

If you click the SEC_ERROR_UNKNOWN_ISSUER code on the error page, it should open a panel with certificate information. The encoded certificate looks like a large block of gibberish. If you see that, please click the button to copy it and then paste it into a reply. Then we can decode the certificate and get a better idea of what's going on.

If you do not get a large block of gibberish when clicking that link:

You may need to investigate using a different site that is a little less strict with its security. For example:

https://www.userchrome.org/

Do you get the error there?

Assuming so: click the Advanced button, then click the "Add Exception" button (see first screenshot below). We're not really going to add an exception, but we can get a better view of the certificate here.

In the Add Exception dialog, click the "View" button to pop up a certificate viewer. The interesting part is the "Issued by" section. I've marked that on the second screenshot for reference.

On my site, the "Issued by" section shows me "Let's Encrypt Authority X3". What do you see there?

(These example screenshots are from a test page, not my site)