Popular sites steal Your logins and passwords from build-in password managers

More info here:
https://www.wykop.pl/link/4088821/popularne-polskie-serwisy-wykradaja-adresy-email-by-sledzic-uzytkownikow/

Demo:
https://senglehardt.com/demo/no_boundaries/loginmanager/index.html

Solutions for Firefox:

about:config
>>> (false)
signon.autofillForms
signon.autofillForms.http
browser.formfill.enable