Do you have a password manager extension that may be suggesting emails and passwords?

I do have a password manager, but it isn't that, as it A: Doesn't suggest passwords in this way B: It was also happening before I had a password manager C: Isn't turned on in private windows, yet I still get suggestions in those windows D: It suggests user details such as emails that are not included in my Password Manager (and never have been), but that I have used before

This could be the "form history" feature. Those look very plain in the drop-down, unlike saved logins.

You can delete individual form history items by using the arrow keys on the keyboard to highlight unwanted suggestions and pressing Delete. You also can stop Firefox saving/suggesting all form and search history. However, I don't think it's possible to make a site exception. More info in the following article:

Control whether Firefox automatically fills in forms