From MozillaZine Knowledge Base
Pressing [Tab] when a web page is in focus will move the focus from element to element. This is helpful when moving to a link you’d like to visit or moving between form fields without using the mouse. This preference determines which elements can be focused by pressing [Tab].
Note: In OS X (as of 2005-01), if this preference is not explicitly set, the “Full Keyboard Access” setting in System Preferences will be honored. All builds before that date (e.g., Firefox 1.0.x) will ignore that setting.
Possible values and their effects
Give focus to text fields only. (The default in OS X.)
Give focus to all form elements except text fields.
Give focus to links (and linked images) only.
You can add any of the above values together to combine their functionality. That is, setting this preference to 3 (i.e., 1 + 2) will let you focus both text fields and other form elements. Outside of OS X, the default value for this preference is 7 (1 + 2 + 4), or focusable text fields, form elements, and links.
Note that at the moment, it is impossible to disable tabbing to text fields; even values for this pref act like their corresponding odd value. See the Related Bugs section below.
Two checkboxes labeled “Links” and “Buttons, radio buttons, checkboxes, and lists” are located under “Edit → Preferences → Advanced → Keyboard Navigation → Tab Key Navigation”. They toggle 4 and 2 above, respectively.
First checked in
Has an effect in