Net Applications' December results show Mac OS X surging from just under 8.9 percent in November to the new 9.6 percent mark for the tens of thousands of sites monitored by the web tracking firm.
The figure is an all-time high for Apple and a significant jump from the same period a year before, when the Mac maker held 7.3 percent.
Its iPhone also made significant inroads and claimed 0.44 percent versus 0.37 percent the previous month, and just 0.12 percent in December 2007. The handset still claims the title of the most popular non-desktop operating system and now has more than half the market share of Linux.
Apple's success during the holiday month, as with the month before, once again comes directly at Microsoft's expense. December represented the second month in a row where Windows had less than 90 percent and dropped nearly a full point to 88.7 percent; both Apple's computers and cellphones were responsible for much of the erosion of Windows' share.
Net Applications does caution that December can potentially skew the results. As more people are staying at home or are on vacation, the researchers note, users are more likely to be running Macs and iPhones than the Windows PCs that still rule the business world.
However, Apple has historically maintained or grown its share following the holiday spike and often uses the season as a platform for further gains.