This graph shows the total number of downloads each day since the release on November 9. These 76 days worth of downloads add up to the 20 million total you see in the graph below.
As you can see, we had an amazing first couple of days with about 2 million downloads. The first week after that averaged over 300,000 downloads per day. Since then, we've been moving between 210K and 270K daily averages.
Without better labels, it's not obvious which days are which but I can tell you from my larger version that the dips are happening around Friday and Saturday (weekends) and the peaks are happening mid-week. You can see the double-peaked section in the middle that sticks out some. That's the New York Times ad pulling the downloads up out of the slump they were in for the 10 days before.
Hope you all enjoy the data dump. One final note. These are taken from our total download numbers which includes all of our mirrors and includes all of the locales hosted by Mozilla and our mirror network. Right now I don't have good data on language breakdowns but I'll try to get a post up soon with some of my estimates.