Just press Ctrl or Shift or Alt and make a RightMouseButton click on a word.
Defaults: Shift+right-click is a Wiktionary lookup, Ctrl+right-click is a Wikipedia lookup, Alt+right-click is Uncyclopedia lookup.
And you can do that over text selected by the mouse. For example, you can highlight DNA repair with your mouse and look it up by pressing Ctrl and RightMouseButton click over the selection.
Left-click on "A"-style icon
in the the bottom right corner of Firefox/SeaMonkey/Flock to toggle WikiLook on/off.