In Clojure the naming convention is roughly similar to camelCase, only that you use a dash "-" instead of an uppercase letter.
fx:id="firstNameColumn" would be ==> fx:id="first-name-column".
I noticed that this FXML file loads without problems:
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<HBox>
  <VBox>
    <Label text='Benutzername:'/>
    <Label text="Passwort"/>
  </VBox>
  <VBox>
    <TextField id="tf-username"/>
    <PasswordField id='pw'/>
  </VBox>
  <Button text="Click Me!" id='bt-1'/>
</HBox>

But when I prefix the "id" with "fx:" then it doesn't work. <TextField fx:id="tf-username"/> ==> Exception.
Please let the parser accept such inputs too.