
Though Microsoft is totes psyched (and they should be) about this turn of events, the record holder for most downloads in 24 hours has been rock solid for almost three years, and that blue ribbon will remain with Mozilla's Firefox 3, released June 17 of 2008.
Mozilla is poised to release Firefox 4 next tuesday, and already enjoys a very large percentage of worldwide browser market share, according to this graph from StatCounter:

Internet Explorer still has the lead, but not by a steady enough margin for it to be the only horse in this race. Between the upcoming Firefox 4 and the already available Chrome 11 Beta, IE9 could very well be brought down in the coming months by a different champion.