Firefox 3.5 reached 5 million downloads in the first 24 hours according to Ars Technica:
Mozilla officially released Firefox 3.5 on Tuesday. The new version of the popular open source web browser has attracted considerable attention and is already seeing rapid adoption. It was downloaded over 5 million times during the first 24 hours. This falls short of the record-setting 8 million downloads that Firefox 3 had during its first day, but it still reflects the intense enthusiasm of the browser’s fans.
The collective number of total Firefox downloads exceeded 500 million last year and is currently estimated at roughly 950 million. It could exceed one billion by the end of August.