Customization
On web pages, by default tabbing navigates through form fields (such as text entry) and anchors (such as links) in the order they appear in the character stream (i.e., in the raw HTML). This can be overridden by the page author using the tabindex attribute, or by disabling a form field.
At the client end, Mac OS X defaults to only focusing text boxes and lists, but this setting can be overridden to allow navigation to all entries (see references). Mozilla Firefox can similarly be customized to include or exclude navigation to text areas, other form elements, and anchors.
Read more about this topic: Tabbing Navigation