Definitely not a FireFox issue. The 502 Bad Gateway error message was a bit misleading and, initially, made me suspect our pfSense appliance.
I did some further testing and discovered that my host computer would not communicate with the outside world using command-line utils such as:
ping
tracert
nslookup
From the ping command, I received the following error code:
Ping Transmit Failed Error Code 1231
I found the following Microsoft article on how to reset the TCP/IP stack due to this error code:
https://answers.microsoft.com/en-us/windows/forum/all/ping-transmit-failed-error-code-1231-windows-vista/0b3216d3-481e-43ca-b222-e55faf56cac2
So, I issued the commands from the article, then re-booted the computer. FireFox now successfully accesses the pfSense areas:
Boot Environment
User Management
without the getting 502 Bad Gateway error. I have no idea how the TCP/IP stack on my host computer got corrupted and why the corruption only affected FireFox and not Chrome...???
Thank you everyone, for your help!