JAWS and menu-button

I'm the developer of the add-on Saved Password Editor ( 
https://addons.mozilla.org/addon/saved-password-editor/ ). I just 
received a report from a user who said that he is unable to access a 
certain drop-down menu. The control in question is a menu-button. Here 
are relevant excerpts of the XUL:

<commandset>
  <command id="edit_signon"
           label="&editentry.label;" accesskey="&editentry.accesskey;"
           oncommand="spEditor.editSignon();" disabled="true"/>
  <command id="clone_signon"
           label="&cloneentry.label;"
           accesskey="&cloneentry.accesskey;"
           oncommand="spEditor.cloneSignon();" disabled="true"/>
  <command id="new_signon"
           label="&newentry.label;" accesskey="&newentry.accesskey;"
           oncommand="spEditor.newSignon();"/>
</commandset>
...
<hbox id="SignonViewerButtons">
  <button id="speMenuBtn" type="menu-button" command="new_signon"
          icon="add" insertafter="removeAllSignons">
    <menupopup id="speMenuBtnPopup">
      ...

He said that when he tabs to the dropmarker, JAWS tells him to press 
Space and then use the up- and down-arrow keys, but that pressing Space 
has no effect. I don't believe I have done anything to negatively 
impact accessibility; the only thing my script does to it is change the 
command and icon of the button. Could I have, in fact, done something 
wrong? Is Gecko doing something to confuse JAWS? Or is there a bug in 
JAWS itself?

Anything you can tell me about this would be appreciated. Thanks.