Even more troubling for Microsoft is IE's quickening decline. IE lost an average of 0.94 of a percentage point in each of the last six months of the year, nearly triple the 0.36 of a point average during the first six months. Notably, the slump came in the face of the availability of IE8, which went final last March, showing that -- at least so far -- Microsoft has been unable to stanch IE's losses.

Microsoft continues to make headway in its campaign to convince users to for IE8, however. When Net Applications accounts for IE8's "compatibility view" -- a feature that lets users display sites as rendered by the older, and often Web standard-incompatible IE6 and IE7 -- Microsoft's newest browser owned a 23.7% share, compared to IE6's 21% and IE7's 15.5% shares.

December marked the first time that IE8 was the most-used Microsoft browser. When the compatibility view data is included, IE8 accounts for 37.8% of Microsoft's total browser usage share. IE6, previously Microsoft's No. 1 edition, fell to the second spot with 33.5% of IE's total.

IE7's number fell farther than IE6's last month -- the former dropped 1.3 percentage points, the latter, 1.1 points -- additional proof that IE8 has stolen much of its share from the more modern IE7. Since IE8's March launch, IE6's share has declined 10.4 points, but IE7 has lost almost twice that, falling 19.6 percentage points in the same period.

The relentless decline of IE has been a boon to rivals, of course, which have collected new users at Microsoft's expense. The shares posted by Safari, Chrome and Opera were all records, for example.