Excellent. Just what I needed. Rated 5 out of 5 stars
There used to be a similar add-on that the developer stopped supporting (since FF4). I have really missed it.
When reading a web page, I often want to look up something in Wikipedia, _without_ interrupting the flow of what I'm currently reading, i.e., I want the look-up to happen in the _background_, so I can read the wp article at a later stage.
There are a number of add-ons that look up Wikipedia, but none of them (as far as I know) do it in a background tab. All the others I have tried interrupt (visually) the page you are currently reading.
Thanks for this. Hope you keep it updated to be compatible with future releases of Firefox.