On the latest StatCouter data, Firefox 3.5 has just crossed the worldwide browser market share barely crossing both IE7 and IE8, standing at almost ~22% market share.
Source: StatCounter Global Stats Browser Version Market Share
However, the US internet users dont share same browser preference as the rest of the world. IE8 still dominates the browser market here in US, but not by much (only ~3%). I think it is fair to say that by the first quarter of next year, Firefox 3.5 should be the leading browser in US.
