Hmm, if they banned Windows 7, they probably would have mentioned that. Maybe it's one of your privacy/security add-ons? One method to suss out an issue with settings or add-ons is:

New Profile Test

This takes about 3 minutes, plus the time to test your problem site.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Test2023, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Test2023 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

Can you access the bank site in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2023 will remain available for future testing.)

Try clearing site cookies and disabling Tracking Protection for the site.

Does it still happen in Troubleshoot Mode?

Somehow the problem seems to have resolved itself. I had contacted my bank around the same time that I contacted Mozilla Support so, perhaps, their IT people figured it out.

Thanks!

As a test I tried logging in with Google Chrome and was successful so it is likely something with the last update of Firefox. Note that I am using Windows 7 and do not want to change to a newer version.

Firefox 116.0 and later requires Windows 10 or later to run so the most current you can use is Firefox 115 ESR currently at 115.8.0esr and the last update will be 115.15.0esr in September 2024. https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support

Chrome already dropped support of the EOL Windows 7, 8, 8.1 back in Jan/Feb of 2023.