Yesterday was Christmas.
In the U.S. Christmas is a holiday for all federal employees, most state and local government employees, and it's the most observed holiday for private business.
So, it is probably a safe bet, though I don't have any data to cite, that there are more people at home on Christmas day in the U.S. than on any other day of the year. I expect that the U.S. isn't terribly different from the rest of the West in this.
So, what does that have to do with Firefox? Well, Firefox does particularly well on weekends and evenings -- when more people are at home (and fewer people are at work using I.E.) So, as you'd expect, Christmas day is a big day for Firefox usage.
How big a day? Well, according to Net Applications' Market Share survey, on Christmas day (measured in U.S. Eastern time) one 1 of every 4 internet sessions came from Firefox users.
At 25% of Internet usage, that's Firefox's best day on record. With the weekend and the "lull" days between Christmas Eve and New Year's Day, I expect we'll see a number of additional strong days of usage as we wrap up 2008. November and December are always strong for Firefox, so I have high hopes we'll wrap the year on a great note.
I'm working on a year end Firefox market share measures post that will have more exciting information. Depending on how much or how little time I spend at the computer, I'll have it posted right before or on New Year's Day.
While your home-vs-office theory is compelling, this could just mean that Firefox use is high among heathens who don't celebrate Christmas, or young people who are less inclined to stay away from the computer on a family-oriented holiday.