Behold, Dictionary Search! This Firefox addon adds other "Search for" options on your context-sensitive menu (the one that comes up with you right-click your mouse). Dictionary Search allows you to highlight a word on a web page, right click it, and search for that word or phrase on any custom site you choose. Don't be fooled by the addon's academic (and thus, to me, scary) sounding name. You don't need to search a dictionary website; you can choose Wowhead as one of your four customizable search "dictionaries."
Read more about how to install Wowhead on the Dictionary Search addon for Firefox.