Here are a few steps you can take to try to solve the problem:

1. Clear the cache and cookies in your browser: Go to "Options" > "Privacy & Security" and click on "Clear Data". 2. Disable browser extensions: Some extensions can interfere with the website's functionality. To disable extensions, go to "Add-ons" and uncheck each one individually. 3. Examine for compatibility issues: Check the website's support page to see if there are any known issues with the most recent version of Firefox. 4. Access the website in a private window to see if the problem is related to your browser settings or extensions. To open a private window, select "New Private Window" from the three horizontal lines in the upper-right corner of the browser. 5. Disable IPv6: Some users have reported that disabling IPv6 solved the "HTTP 400 Bad Request" error. To do this, type "about:config" in the address bar, search for "network.dns.disableIPv6", and set the value to "true".