Firefox prevents me from going to a website I have gone to for years - Error 400 comes up.
I go to many websites during the day and never have a problem.
I go to Schwab.com and a Firefox tells me there is a problem with that website (not according to Schwab, where I have been a client for 30 years).
It mentions something about a "certificate requirement" and I go ahead and give Firefox approval to access Schwab
Try to rename the cert8.db file in the Firefox profile folder to cert8.db.old or delete the cert8.db file to remove intermediate certificates that Firefox has stored.
If that helped to solve the problem then you can remove the renamed cert8.db.old file.
Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previous intermediate certificates.
Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.
You can use this button to go to the Firefox profile folder:
Help > Troubleshooting Information > Profile Directory: Show Folder