[redthrowaway]

I'm thinking FF 3.6, which came out January 21st. [1] If you zoom in on 2010, you'll see the drop happens around the end of January. [2]

[1] http://www.mozilla.com/en-US/firefox/3.6/releasenotes/

[2] http://trends.google.com/trends?q=facebook.com&ctab=0&#3...