Some of the recent losses are being attributed to the diminished usage of Internet Explorer in the workplace. While we've been calling IE to be broken for years now, typically businesses and institutions adopt new software more slowly.
The losses IE has suffered amount to around 10% for the whole year and represent gains across the board for others, namely Firefox, Safari and Chrome. All three of these browsers have gained in December with Firefox exceeding 21% and Chrome coming in above 1%. These aren't localized figures, and we've seen statistics in the past that show actual usage of browsers can vary wildly by what region of the world people are in. Another very worthy competitor Opera seems to be flat in terms of market growth.
For now it's clear that IE keeps falling and only a drastic move from Microsoft will keep this trend from continuing. So far we've seen none.