From MozillaZine Knowledge Base
Originally for debugging purposes, this preference now determines whether or not Caret Browsing mode is active. Caret Browsing is provided as a way to navigate through a web page—using only the keyboard—by moving a text caret. This mode is more easily toggled by pressing [F7].
Caret Browsing mode changes the effects of the arrow keys, as well as the [Home] and [End] keys.
Possible values and their effects
A blinking cursor (the caret) is visible on all displayed webpages, allowing the user to navigate through the page as they might a word processor document.
The system caret is hidden. (Default)
Tools → Options → Advanced → General / Accessibility → "Always use the cursor keys to navigate within pages" (Firefox 2 and above)
First checked in
Has an effect in