There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

I'm running Windows 10 with no other AV than the integrated one, so your answer is not my case...

Is there any information that I can try to get you to try to troubleshoot this issue?

Did you check the links I posted?

Yes. None worked. I even tried disabling DNS over HTTPS, still nothing.

This is the exact message:

Secure Connection Failed

An error occurred during a connection to www.google.com.

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
   Please contact the website owners to inform them of this problem.

I'm only having problems with Google related pages. Google calendar fails sometimes, but Google Maps almost 100% of the times.

Hope it helps!

could you enter about:config into the firefox address bar (confirm the info message in case it shows up), search for the preference named network.http.http3.enabled and check that it is set to false?

It was set to true. I must have set it to true manually time ago (at least a couple of months back). Strange that it was working fine before but not anymore... Disabling http3 makes it work perfectly again.

Thanks!

PD: Will try to enable it back again in the future. Will try maps, seeing that it's the most affected by it.