Hi raebc, I think that could also be caused by Firefox sending the site cookies it previously set but which it no longer likes for one reason or another. Here's how to clear those:

While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "Clear Cookies and Site Data"
• (menu bar) Tools > Page Info > Security > "Clear Cookies and Site Data"

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

You also could try testing in Firefox's Safe Mode. In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

In addition to the above, please see :

https://www.lifewire.com/how-to-fix-a-400-bad-request-error-2617988

Thank you so much for your quick response! The first solution, "Clear Cookies and Site Data" worked!!

The same solution also worked for an issue I'd been having with the Cox Communications site for MONTHS. Thank you again!