As initially noted on the user@ mailing list, connecting using Firefox occasionally results in the connection behaving as if the return/enter key is being held down, even though the key has not been pressed locally.
This behavior is actually due to Firefox not sending the keyup event when the login form is submitted using the "Enter" key. As the Guacamole.Keyboard instance is at the index level, present even before the user views a client page, it receives the keypresses which occur during login, including when the form is submitted. Lacking a keyup event for "Enter", the Guacamole.Keyboard instance assumes the key is held down and continually repeats the key. Once the user views a connection, those repeated enter keypresses are forwarded to the remote desktop.