If you create an .htm file containing the following HTML (which was created by the textbox button on your toolbar) and just open that file in Firefox, you can type into the textbox. But, you cannot type into the textbox inside your editor. So, it would seem that this inability to type into a form element like a textbox is not default Firefox behavior, but something related to your control.
<input tabindex="1" maxlength="20" size="10" name="text1" id="text1"
align="bottom" type="text" />