Form select can end up disabled in Firefox

I haven't figured out exactly what is causing this - but when I put a Hierarchical Select field at the bottom of the form, it works fine - however if I move it to the top the select box ends up disabled in Firefox (it works fine in Chrome). The select box has disabled="" in firebug, so the code is obviously running, however the browser is not registering the change. My guess is this is some race condition. Removing the "disabled" attribute (rather than setting it to false) appears to work fine however - I think this must trigger a different DOM event.