Firefox sometimes remembers a previous failure. To bypass a cached error page, you can try reloading using Ctrl+Shift+r to force a full reload.

If the load still fails, can other applications successfully access the internet from the same device?

I've tried the CTRL+SHIFT+r to force reload and that didn't make a difference. Yes, other applications and other devices work. I've tried this among other places Chicago O' Hare ATT and Admiral's Club. Previously they used T-Mobile and that worked. When ATT took over, it stopped working. Indiana International in Indianapolis, changed from a free WIFI to AWG. There I get the sign-on page, enter my information and then get "Page not found". It appears that any system that uses a captive DNS and / or page redirect will fail to be processed by Firefox. Consistently. I am using the Symantec End Point security. I have shut that down, which does not seem to make a difference. When I say shut down, I mean that I've disabled functions, not removed it from the system. Attached is how the End Point security is set when I try to log into WIFI.

You should not need to disable antivirus to connect.

I assume you are not having Firefox block all redirects. You can check that setting here:

orange Firefox button or classic Tools menu > Options > Advanced > General > uncheck "Warn me when websites try to redirect or reload the page"

What are your connection/proxy settings in Firefox? If you choose "Use system proxy settings" then Firefox should use IE's settings. Or if you choose "No proxy" then Firefox should default to your wi-fi connection.

orange Firefox button or classic Tools menu > Options > Advanced > Network > "Settings" button

Can you think of any add-on changes you might have made since the T-Mobile days? A standard diagnostic to bypass interference by extensions (and some custom settings) is to try Firefox's Safe Mode.

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode) using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode."

If you can load sites normally, this points to one of your extensions or custom settings as the problem.