Since there is no URL for anyone to test on their Firefox browser no one can know what is happening here. And no one two sites interact the same with the same Browser. And screenshot helps other to compare if they get the same issue trying to get to the site.

Thanks for getting back to me. Unfortunately the site in question is a payroll site, and I don't feel comfortable to putting the URL on a public forum. Suffice to say the URL is nothing more than corporatename.com/setup, which is why it mystifies me that I keep getting the message saying the header field is over the server limit. Plus, I've accessed the site for years with Firefox and the exact same URL with no problems. That's why I'm wondering if there's a way to override the browser, like you can when it blocks a site it deems unsafe, but you can put it on a list of approved sites.

Then that to me means that the site itself security checks is preventing Firefox from properly loading. I've seen sites like Business and Financial institutions have secure checks to insure the browser used matches their security protocols to allow access to sensitive information and that might be blocking that access. Your next step is to ask the IT security what Browser do they recommend using to access your payroll site securely in the future if Firefox doesn't work. My guess is they will probably tell you to use Edge/Chrome or if iO/S Safari to access that data.

That's good advice -- thank you. I will call the site's IT department today.