Been looking for an add-on to give me a pop-up with definitions from Oxford Dictionaries, this is the closest I've found so far. It's not a pop-up, but it's easy enough. If you want the definition in a new tab on the official website rather than a pop-up, this is for you.

Edit: I just tried the add-on "Pop-Up Dictionary" which seemed promising, but it doesn't work well with oxforddictionaries.com, so I'll be sticking with this.

Edit #2: A problem with this add-on is that it doesn't return to the previous tab you were on after closing the Oxford Dictionaries tab, it goes to the last tab in the window. Dev, hopefully you can fix this.

A pop-up window would be better though