You can set such an exception via the doorhanger attached to the Key icon at the left end of the location/address bar that opens when Firefox offers to store the password.

You can also check this on the about:permissions page although this page will be discontinued in one of the next Firefox versions.

A detail like a website remembering you (log you in automatically) is stored in a cookie.

You can use these steps to make a website recognize and remember you:

• Create a cookie 'allow' exception to keep such cookies, especially in case of secure websites and when cookies expire when Firefox is closed.
• Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

You can remove the cookies from a website to make the website forget about you.

I don't use this feature, but when I have tried it from time to time, one of the options was "Never for this site." Does the prompt on PayPal give you that option?

Another way to access that would be using the "Permissions Manager" page. To open that, type or paste about:permissions in the address bar and press Enter/Return to load it.

The left column will start filling with a list of sites for which Firefox has stored cookies, history, custom permissions, etc. You can type part of the site name (paypal) in the search box above the left column to filter the list. Then click the site and on the right side the first permission should be "Store Passwords" with the options of Allow and Block.

Hopefully then it will show up in the Exceptions list.