XUL:Focus Behaviour

The following is how Mozilla determines what elements can be focused and which can not:

elements that are hidden (display: none) cannot be focused
elements that have the CSS visibility property set to collapse or hidden are not focusable.
an HTML element in an editor is not focusable
an HTML element cannot be focused if it has a disabled attribute.
an XML element cannot be focused unless it has a link/xlink.
a XUL element that implements nsIDOMXULControlElement cannot be focused if its disabled property is true.
otherwise the focusability of an element is determined by the -moz-user-focus property. If it is set to 'ignore' or 'none, the element cannot be focused. Other values allow focus.
the value of -moz-user-focus is overriden for HTML elements if they have a tabindex attribute
the value of -moz-user-focus is overriden for an enabled XUL element that implements nsIDOMXULControlElement if its tabIndex property returns a value greater or equal to zero.
you can focus a scrollable HTML element -- one with overflow set to 'scroll' or 'auto' -- if you change the focus with the tab key, but not by using the mouse.

Some HTML elements such as form elements have additional focus rules.

On the Mac, there's a system setting "Full Keyboard Access", which can be used to toggle between this model and a model where only listboxes, trees and textboxes can be focused.

The following XUL elements implement nsIDOMXULControlElement:

button, caption, checkbox, colorpicker, description, label, listbox, listitem, menu, menuitem, menulist, radio, radiogroup, richlistbox, richlistitem, tab, tabs, textbox, tree

The following XUL elements have -moz-user-focus set to normal:

browser, editor, iframe