The Spread Firefox blog refers to a study done by the folks at Xiti (an analytics firm in France) indicating that in Europe, Firefox has reached 20% market share. It looks to me more like 16.9%, but still impressive, and "ahead" of the U.S. market:
They do note a few important caveats in the study (if I'm reading the translation correctly), including the fact that the research was done on a Sunday, which they contend is a higher Firefox day.
It's not clear to me what types of sites they are collecting data on (ecommerce, media, etc.), and other important factors that might skew the numbers. But if you assumed for the moment that where they suggest the U.S. is at 13%, and we more generally hear it is around 8-10%, then perhaps the 16.9% in Europe might be closer to 11-14% (roughly speaking!).