Using Alt+ArrowDown should open the form comboboxes (as it is standard under Windows and as the UI comboboxes support). (If you are at it, make sure that [tab] on a open combobox closes it)
Setting all/all because this is also a problem on Windows. See also bug 64078, where I suggested that the space key should drop down the menu for drop-down listboxes (I wasn't aware of the F4 and alt-down tricks when I filed that bug). See bug 64079 for making these menus close when you press tab.
See bug 57192 for F4.
Setting to WONTFIX since this will be fixed by the XBL-rewrite of the select.
This is more of a duplicate of bug 57192. Marking it verified.