If you want to push back against the bureaucracy on this one, find a Firefox-only accessibility addon that can’t be used on their site, and play the ADA angle b

If you want to push back against the bureaucracy on this one, find a Firefox-only accessibility addon that can’t be used on their site, and play the ADA angle by sending a polite email mentioning that their negligent browser restrictions prevent “your favorite” visibility tweaker/screen reader/etc. from being used, harming the ability to access the site. You need not disclose the details of exactly what, if any, disabilities you personally suffer from.