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 dom.event.clipboardevents.enabled to False.

In case this setting disappears in a future version of Firefox, it is always possible to deactivate the event through Javascript by targeting specifically the password field. Open the developer console (SHIFT+CTRL+I) and type : document.getElementById("password2").onpaste = null; (in this example, password2 is the id of the password field)