I noticed that in some, but not all WolframAlpha queries several of the "boxes", such as the results box, aren't shown (see the attached screenshots). With the addon disabled, the problem disappears.
A query that shows the problem: https://www.wolframalpha.com/input/?i=+EUR+%2F+m%5E2+in+USD%2Fft%5E2
I'm using Firefox 20.0.1.
Start: None
Due: None