I am using Windows 8 with Firefox 20 and - like seen in older versions - in some cases the select box does not open at all.
The HTML is wrapped by a form, so I do not expect the html to be wrong. Also, I deactivated all add-ons, to be sure they are not causing the issue.
UPDATE: somehow it seems that this happens when the content of the page changes, e.g. i expand a list view above this select box in (own ), that means the position of the select box changed after that it does not react to any interaction anymore