The Bank site that is having the problem is the "Royal Bank" in Canada (Canada's largest Bank)

This issue can be caused by corrupted cookies or blocked cookies.

• check the permissions for the domain in the currently selected tab in "Tools -> Page Info -> Permissions"

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings).

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

If clearing cookies doesn't help, it is possible that the cookies.sqlite file in the Firefox profile folder that stores the cookies got corrupted.

• rename/remove cookies.sqlite (cookies.sqlite.old) and when present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder with Firefox closed in case cookies.sqlite got corrupted.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thank You for your advice - what I did was my usual monthly Cache emptying and Backup a few days early, and it seems to have worked - The Bank site is now working 100%

Correction - It worked properly 100% for most of today and is now doing it again with the "Bad Request" message....

Try clearing the cookies for the website right when you get the message -- click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page and signing in again. Does that help?

Did you once again try to clear the cookies ? Do you have extensions that deal with cookies as it is not normal that cookies get corrupted that often ?

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

Thank you both for your suggestion to clear the cookies - I have now done that and the Royal Bank site appears to be working properly - If you don't get a message from me again, that will prove that your advice worked, as I suspect.