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.
To add Wowhead to your custom dictionaries, install the addon and restart Firefox. Click the Tools menu and choose Add-ons. Scroll till you find Dictionary Search on your list and click Options underneath it. Type in what you see in the red box in the image below.
Now, whenever you want to see something in Wowhead -- no matter what site it's on, even in your email -- all you have to do is highlight the item's name, right click, and there you have it in a new tab under the page you're clicking from. Works with both Firefox 2 and Firefox 3, but hey, upgrade to 3.0 while you're at it, okay? This is your guru talkin', after all.