Stop Notification PopUp "Do You Want to Save Login"

It's Firefox simply way is say yes then it won't ask again

Quite probably a password manager of some sort, whether your browser's or a 3rd party program.

dw85745 said:

...Narrowing down to Firefox helps.

You can turn this off (or on) in Firefox Settings > Privacy & Security

Bear in mind that without further information from you, this could apply to any browser.

Why? For example, 3rd party password managers often support multiple browsers, with an add-on per browser, making the support of automatic fill of fields etc uniform across browsers and even devices (e.g, an Android phone).

We don't know how you have configured your system; whether you are dealing with an individual browser, or are using a password manager. (Which, as you note your concern about security, encrypts said information).