When you press F7 in FireFox, you get the following message:
Pressing F7 turns Caret Browsing on or off. This feature places a moveable cursor in web pages, allowing you to select text with the keyboard. Do you want to turn Caret Browsing on?
Caret browsing allows you to move the caret using the keyboard arrows, so you can select text on web pages without having to use the mouse.