I have recently added Answers 2.3.54 add-on to my FireFox 19.0.2.
This is supposed to provide one with instant information and definitions of any particular word or term, simply by right clicking on the word while holding down the Alt key.
But this only seems to work the first time one loads firefox, but as soon as one changes the page it ceases to work.
I've just tried this myself and you're right it only works once.
I use Wikilook which works in a very similar manner and most importantly it does work. (For me at least)
I would recommend it.