By "standard", I mean Apple's standard interface elements text boxes, menus, button, etc. that are available to developers and behave the same in every program that uses them. Firefox doesn't use any of them, but instead creates its own that look similar but act quite differently. (So that it's more consistent across platforms...or at least easier to port.)
I suppose the reason it's not nagivable with the keyboard is that the keyboard is used for other things. The arrows scroll, for example. Non-editable text fields don't usually get the keyboard focus.
I'm not trying to say it's better this way, just explaining the reason.