Some websites disable password pasting which is extremely annoying when using a password manager like Keepass. The developer (or more likely the client) probably believes it is a security measure althought it is mostly a pain for the user.
Hopefully there is a quick way to work around this in Firefox. Type
about.config in the address bar, then set
document.getElementById("password2").onpaste = null; (in this example, password2 is the id of the password field)