It seems like Firefox has a problem with disabled and re-enabled text input fields. Once deactivated and then reactivated, the blinking cursor disappears. It is also not possible to see the selection. Although everything works just fine, it is pretty annoying. Here is the solution for this problem. It involves Javascript, but you probably won’t have this problem when you don’t use Javascript to de- and re-activate text fields:
var inbox = document.getElementById( 'idOfInputBox');
inbox.blur();
inbox.disabled=false;
inbox.focus();