Sounds like an add-on issue, but your screenshots are not showing the browser window, only what's in the browser which is why I don't like the screenshot option in Firefox. END OF RANT  ;-))

Did you try with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Did you clear your browser cache? https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox

Are you running any security software besides MS Defender? Windows home or pro? If Windows Pro version, try testing in Windows Sandbox (VM) to see if you have the same issue. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

The site loads within 1sec for. see screenshot

First, thank you for your response. I apologize for my delay answer, but things have been a little hectic recently. I tried your suggestions but without success. I’ve been having several, seemingly related problems over the last month or more. Finally I contacted a technician from the company that provides my internet access. He changed a couple of old connections and replaced my router/modem with a new one. He said the problem was not my internet access. I then brought my computer to a specialist who told me that the problem was likely a hardware issue and would likely be difficult/expensive to fix. So I took the plunge and bought a new, W11 computer. The transition to the new computer is slow and not without a few problems (mostly my errors). After a short struggle, I got Firefox to work but the transition of Thunderbird went very smoothly. So now I am continuing the process of moving and converting. I hope I don’t have to return here for additional assistance, though I know there are many helpers out there. Thank you again.