Some people complained about my graphic in the previous post so I've tried again. I hope it's clearer than the last one.
And some other folks have asked more about the holiday dip for Firefox so I'll take a second to offer my best explanation for this and why Chrome doesn't see it.
Firefox has nearly 400 million users this month. That's a lot of people and it includes hundreds of millions of "regular people" who don't live their entire lives online. During the winter holidays there are a few weeks around Thanksgiving, Christmas, and New Years where these "regular people" abandon their computers and spend time with friends and family doing things like shopping and eating and having fun.
When Firefox was still an "alternative" browser with just a few tens of millions of users, a much larger percentage of them were "power users" like you and me who live our lives online and so this drop-off was a lot less recognizable and more localized to the actual holiday days (even a geek has to go out with friends on new years eve, right?)
Chrome is still dominated by the early adopter, power user crowd that spends more time online than offline and that accounts both for the disproportionately large usage:users ratio as well as the lack of a significant dip during times with regular people do leave their computers.
That's my take on it.