PDFs no longer display in Safari, billing sites say Adobe is required, yet same PDFs open in Firefox

As you already know, the billing site is wrong.

It is certainly possible Safari is unable to open that particular PDF though. Is Safari able to download others? Use this as an example (it's a small, one page document):

https://www.apple.com/environment/pdf/Apple_EHS_Policy_Statement_2022.pdf

The following may be relevant: If Safari doesn't open a page or work as expected on your Mac - Apple Support. There are a lot of suggestions in that document. Some of them may be helpful.

Adobe can make changes to their PDF specifications at will, so all everyone else can do is try to keep up.

There probably is no setting you can change. Just keep macOS updated.

Firefox (and Brave, for that matter) continue to provide support for their browsers long after Apple stops supporting Safari.