I finally found the problem and the solution.
The problem is I have an incorrect system clock.
The solution is I automatically set my desktop time zone setting.
I read this guide and it has a topic about incorrect system clock.
Firefox uses certificates on secure websites to ensure that your information is being sent to the intended recipient and can't be read by eavesdroppers. An incorrect system date can cause Firefox to detect that the website's security certificate is expired or invalid. Make sure your computer is set to the correct date, time and time zone. For more information, see How to troubleshoot time related errors on secure websites.
You can also read this guide
https://support.mozilla.org/en-US/kb/secure-connection-failed-firefox-did-not-connect?as=u&utm_source=inproduct
Now, I can use phpmyadmin SSL correctly. It also shows correct SSL expiration date when accessing the phpmyadmin public access URL through a browser. It may be related to when recently changed my motherboard CMOS battery because I replaced a non working CMOS battery.
Thank you for your help.