First, to comment on some nice things in this add-on.
-Useful and functional add-on, while also providing a clean look
-I like the bar at the top. I also really like how when using the back and forward buttons, the page changes instantly. Faster than it does through the web browser. This might be because of the many add-ons I have installed and they are not enabled within your add-on.
I don't like how it opens in its own type of window.
-This is not preferable for me because I am reverted to the default smooth scrolling in Firefox, and I like using my SmoothWheel add-on.
-By opening Wikipedia in it's own type of window, it is not saved in the history of Firefox. So there could be options for the user in the settings to do this (for people who like choice), I think it would good for there to be an option to save the Wikipedia results by default, and not the dictionary results by default.
I'm saying "it's own window" because I don't know how else to describe it. But another add-on that I use called FoxLingo that functions similarly to QuickWiki and opens in a window that does not look like a Firefox window, does not have the problem that it does with the SmoothWheel add-on.
And personally, I prefer the FoxLingo add-on for wiktionary lookup because it provides me with other dictionary results as well, since I see others have commented on that. This is useful because FoxLingo puts a different dictionary result at the top when there is no dictionary entry for wiktionary, which is also when QuickWiki does not work.